Use an older version of RubyGems on Travis to fix 1.8.x issue

The Travis build is currently erroring due to an issue with RubyGems and 1.8.x/JRuby 1.8.x compat mode

https://github.com/bundler/bundler/issues/2784

This moves back to a version of RubyGems that works until a new version is released. Everything is back to green on my fork now

https://travis-ci.org/abedra/brakeman