Add syntax highlighting for Module#refine and Module#using

This adds syntax highlighting for [refinements](http://www.ruby-doc.org/core-2.1.0/doc/syntax/refinements_rdoc.html) in new versions of Ruby. Specifically, this will highlight [Module#refine](http://ruby-doc.org/core-2.1.0/Module.html#method-i-refine) and [Module#using](http://ruby-doc.org/core-2.1.0/Module.html#method-i-using).