Bump to Rails 4.2.0.rc release line.

Locking `arel` is no longer necessary as the RC1 release has the proper
fixed arel call. However, it requires `rails-dom-testing` 1.0.5+ which
fails for us on Travis CI (linux). This allows the Rails 4.2.0 envs to
fail without killing the build, until `rails-dom-testing` is fixed.

See https://github.com/rails/rails-dom-testing/issues/28