Stop testing with JRuby, but start testing with REE

It seems like that JRuby on travis-ci is broken[1], so we shouldn't test against it to reduce the false-positive result. I've run the test locally, and it passed just fine[2].

[1]: http://travis-ci.org/#!/thoughtbot/paperclip/builds/79181
[2]: https://gist.github.com/299686b412f9ad878e16

.travis.yml

 rvm:
   - 1.8.7
   - 1.9.2
-  - jruby
+  - ree
 script: "bundle exec rake clean test"
 gemfile:
   - gemfiles/rails2.gemfile