remove duplicate faraday dependency

Bundler 1.6.0 fails to install berkshelf v2.0.16 due to a duplicated `faraday` dep in the gemspec.  This PR removed the duplicate line, and set the dependency to the highest version of the two existing ones (`0.8.5`)

```
ERROR:  While executing gem ... (Gem::InvalidSpecificationException)
    duplicate dependency on faraday (~> 0.8.0), (~> 0.8.5) use:
    add_runtime_dependency 'faraday', '~> 0.8.0', '~> 0.8.5'
	from /opt/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/bundler-1.6.0/lib/bundler/gem_helper.rb:57:in `build_gem'
	from /opt/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/bundler-1.6.0/lib/bundler/gem_helper.rb:39:in `block in install'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/rake-0.9.6/lib/rake/task.rb:228:in `call'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/rake-0.9.6/lib/rake/task.rb:228:in `block in execute'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/rake-0.9.6/lib/rake/task.rb:223:in `each'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/rake-0.9.6/lib/rake/task.rb:223:in `execute'
	from /var/cache/omnibus/src/berkshelf/Thorfile:13:in `build'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/command.rb:27:in `run'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/runner.rb:36:in `method_missing'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/command.rb:29:in `run'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/command.rb:128:in `run'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /opt/opscode-analytics/embedded/lib/ruby/gems/2.1.0/gems/thor-0.18.1/bin/thor:6:in `<top (required)>'
	from /opt/opscode-analytics/embedded/bin/thor:23:in `load'
	from /opt/opscode-analytics/embedded/bin/thor:23:in `<main>'
```