require set

from our CI logs

```
uninitialized constant Syntax::Ruby::Set (NameError)
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/syntax-1.2.0/lib/syntax/lang/ruby.rb:12:in `<class:Ruby>'
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/syntax-1.2.0/lib/syntax/lang/ruby.rb:8:in `<module:Syntax>'
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/syntax-1.2.0/lib/syntax/lang/ruby.rb:3:in `<top (required)>'
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/syntax-1.2.0/lib/syntax.rb:24:in `load'
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/syntax-1.2.0/lib/syntax/convertors/abstract.rb:16:in `for_syntax'
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/cucumber-1.3.14/lib/cucumber/formatter/html.rb:580:in `<class:SnippetExtractor>'
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/cucumber-1.3.14/lib/cucumber/formatter/html.rb:578:in `<class:Html>'
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/cucumber-1.3.14/lib/cucumber/formatter/html.rb:8:in `<module:Formatter>'
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/cucumber-1.3.14/lib/cucumber/formatter/html.rb:7:in `<module:Cucumber>'
/Users/ci/.jenkins/workspace/soundcloud_rac_master/vendor/bundle/ruby/2.0.0/gems/cucumber-1.3.14/lib/cucumber/formatter/html.rb:6:in `<top (required)>'
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
```