Revert Gemfile back to Rails 3.0.X

This reverts problematic Fedora 18 upgrade. I kept one commit that avoids one specific gem version (ZenTest  4.8.4) which contains parsing error in the gemspec.

After this PR is merged, Gemfile is back to Rails 3.0 and bundler works again. Tomorrow we can plan how to upgrade properly and I vote for doing this in a separate branch if possible.

Merge at will.