loose dependencies on other cookbooks

Allow new-er versions of `build-essential` and `postgresql` cookbooks to co-exist with `sphinx`. The strict dependency rules cause tools like `librarian-chef` to fail when this cookbook is used with other cookbooks (like `nginx`) that require higher versions of `build-essential`.

Example error:
```
/...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/lib/librarian/chef/source/site.rb:309:in `unpack_package!': The package archive has too many children! (RuntimeError)
        from /...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/lib/librarian/chef/source/site.rb:224:in `cache_version_uri_unpacked!'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/lib/librarian/chef/source/site.rb:93:in `block in version_uri_manifest'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/lib/librarian/chef/source/site.rb:378:in `memo'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/lib/librarian/chef/source/site.rb:92:in `version_uri_manifest'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/lib/librarian/chef/source/site.rb:88:in `version_manifest'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/lib/librarian/chef/source/site.rb:54:in `version_dependencies'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/lib/librarian/chef/source/site.rb:459:in `fetch_dependencies'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/manifest.rb:121:in `fetch_dependencies!'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/manifest.rb:113:in `fetched_dependencies'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/manifest.rb:77:in `dependencies'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:131:in `sourced_dependencies_for_manifest'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:60:in `block in recursive_resolve'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:143:in `block (3 levels) in resolving_dependency_map_find_manifests'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:176:in `block in scope_checking_manifest'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:208:in `scope'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:175:in `scope_checking_manifest'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:142:in `block (2 levels) in resolving_dependency_map_find_manifests'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:196:in `block in map_find'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:195:in `each'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:195:in `map_find'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:141:in `block in resolving_dependency_map_find_manifests'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:154:in `block (2 levels) in scope_resolving_dependency'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:168:in `block in scope_checking_manifests'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:208:in `scope'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:167:in `scope_checking_manifests'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:153:in `block in scope_resolving_dependency'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:208:in `scope'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:152:in `scope_resolving_dependency'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:140:in `resolving_dependency_map_find_manifests'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:56:in `recursive_resolve'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver/implementation.rb:32:in `resolve'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/resolver.rb:16:in `resolve'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/action/resolve.rb:26:in `run'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/cli.rb:169:in `resolve!'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/lib/librarian/chef/cli.rb:41:in `install'
        from /...x.../bundle/ruby/2.0.0/gems/thor-0.18.1/lib/thor/command.rb:27:in `run'
        from /...x.../bundle/ruby/2.0.0/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
        from /...x.../bundle/ruby/2.0.0/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
        from /...x.../bundle/ruby/2.0.0/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/cli.rb:26:in `block (2 levels) in bin!'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/cli.rb:31:in `returning_status'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/cli.rb:26:in `block in bin!'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/cli.rb:47:in `with_environment'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-0.1.1/lib/librarian/cli.rb:26:in `bin!'
        from /...x.../bundle/ruby/2.0.0/gems/librarian-chef-0.0.2/bin/librarian-chef:7:in `<top (required)>'
        from /...x.../bundle/ruby/2.0.0/bin/librarian-chef:23:in `load'
        from /...x.../bundle/ruby/2.0.0/bin/librarian-chef:23:in `<main>'
```