Bump rails 3_1 version to rc6

This reverts commit fe01ae69.

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

Update Gemfile to make sure we can run the test locally, and also upgrade Rails 3.1 beta to latest version

Closes #534, closes #538

We already merged it in 6a10b358 and 0708cd43

This reverts commit ff3929ee.

Add ability to specify ".allowing" and ".rejecting" individually with content_type validation matcher.

Closes #472, Closes #532

This will fix the problem with Psych on the old version of Rails 2.x

Only notify @sikachu for build result for now. Will turn this off after they add the Campfire notification