Lock soloist to < thor 0.18.0

This happens with thor 0.18.0

```
graystone:~ pivotal$ soloist
/Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/chef/manifest_reader.rb:49:in `binread': undefined method `binread' for #<Pathname:0x106812db8> (NoMethodError)
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/chef/manifest_reader.rb:19:in `read_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/chef/source/site.rb:96:in `version_uri_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/chef/source/site.rb:340:in `memo'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/chef/source/site.rb:92:in `version_uri_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/chef/source/site.rb:88:in `version_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/chef/source/site.rb:54:in `version_dependencies'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/chef/source/site.rb:421:in `fetch_dependencies'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/manifest.rb:121:in `fetch_dependencies!'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/manifest.rb:113:in `fetched_dependencies'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/manifest.rb:77:in `dependencies'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:131:in `sourced_dependencies_for_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:60:in `recursive_resolve'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:143:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:176:in `scope_checking_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:208:in `scope'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:175:in `scope_checking_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:142:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:196:in `map_find'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:195:in `each'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:195:in `map_find'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:141:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:154:in `scope_resolving_dependency'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:168:in `scope_checking_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:208:in `scope'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:167:in `scope_checking_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:153:in `scope_resolving_dependency'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:208:in `scope'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:152:in `scope_resolving_dependency'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:140:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:56:in `recursive_resolve'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:62:in `recursive_resolve'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:143:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:176:in `scope_checking_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:208:in `scope'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:175:in `scope_checking_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:142:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:196:in `map_find'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:195:in `each'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:195:in `map_find'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:141:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:154:in `scope_resolving_dependency'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:168:in `scope_checking_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:208:in `scope'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:167:in `scope_checking_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:153:in `scope_resolving_dependency'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:208:in `scope'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:152:in `scope_resolving_dependency'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:140:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:56:in `recursive_resolve'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:62:in `recursive_resolve'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:143:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:176:in `scope_checking_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:208:in `scope'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:175:in `scope_checking_manifest'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:142:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:196:in `map_find'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:195:in `each'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:195:in `map_find'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:141:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:154:in `scope_resolving_dependency'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:168:in `scope_checking_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:208:in `scope'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:167:in `scope_checking_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:153:in `scope_resolving_dependency'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:208:in `scope'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:152:in `scope_resolving_dependency'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:140:in `resolving_dependency_map_find_manifests'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:56:in `recursive_resolve'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver/implementation.rb:32:in `resolve'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/resolver.rb:16:in `resolve'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/action/resolve.rb:26:in `run'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/cli.rb:168:in `resolve!'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/chef/cli.rb:41:in `install'
	from /Library/Ruby/Gems/1.8/gems/soloist-1.0.0/lib/soloist/cli.rb:37:in `install_cookbooks'
	from /Library/Ruby/Gems/1.8/gems/librarian-0.0.26/lib/librarian/cli.rb:47:in `with_environment'
	from /Library/Ruby/Gems/1.8/gems/soloist-1.0.0/lib/soloist/cli.rb:36:in `install_cookbooks'
	from /Library/Ruby/Gems/1.8/gems/soloist-1.0.0/lib/soloist/cli.rb:35:in `chdir'
	from /Library/Ruby/Gems/1.8/gems/soloist-1.0.0/lib/soloist/cli.rb:35:in `install_cookbooks'
	from /Library/Ruby/Gems/1.8/gems/soloist-1.0.0/lib/soloist/cli.rb:16:in `chef'
	from /Library/Ruby/Gems/1.8/gems/thor-0.18.0/lib/thor/command.rb:27:in `__send__'
	from /Library/Ruby/Gems/1.8/gems/thor-0.18.0/lib/thor/command.rb:27:in `run'
	from /Library/Ruby/Gems/1.8/gems/thor-0.18.0/lib/thor/invocation.rb:120:in `invoke_command'
	from /Library/Ruby/Gems/1.8/gems/thor-0.18.0/lib/thor.rb:363:in `dispatch'
	from /Library/Ruby/Gems/1.8/gems/thor-0.18.0/lib/thor/base.rb:427:in `start'
	from /Library/Ruby/Gems/1.8/gems/soloist-1.0.0/bin/soloist:4
	from /usr/bin/soloist:19:in `load'
	from /usr/bin/soloist:19
```