Fix bundle update on the Gemfile in travis.

I didn't have this problem locally, but for some reason the other Gemfile's
have this dependancy on the minitest gem and don't have the same problem.

Gemfile

@@ -3,3 +3,5 @@ source "https://rubygems.org"
 gemspec
 
 gem "activeresource", '~> 4.0.0'
+
+gem 'minitest', "~> 4.2"