added support for rubies > 1.9.3

ruby_parser changed how they handle module constants -- parse tree now looks like: 
```s(:colon2, s(:const, :Encoding), :UTF_8)```. 
This PR fixes the issue. 

Note that I removed Gemfile.lock, as it was hiding the issue above (https://github.com/seattlerb/ruby_parser/commit/0a9da94a457a7ef5e2c8cb3f865f57439cd5bd23 was committed more than a year ago). Not sure if it's worth pinning minor versions of dependencies in the Gemfile -- I only updated existing constraints.