No need to require mocha/test_unit with mocha/setup
Bump Rails 2.3/3.0 versions to latest and use Yajl as JSON backend

The Rails implementation that uses OkJson simply fails the key value
tests we have, whereas Yajl seems to work fine.

I'll leave Yajl for now to have the tests passing on latest 2.3/3.0
versions, and we can get rid of all this after removing support for
older Rails versions.