Start Running tests on Stable JRuby

Fixes for LOT of bugs have now landed in Jruby 1.7.6, thus we no longer need to run tests against the master.

for broken tests that are still remaning, I think this what we should do:
1. If problem is in rails - we fix that test/code
2. if its a JRuby bug - we skip that test & file a bug report for jruby.


About status of failing tests:

1. ap,am,amo,as - few encoding related tests are the problem. otherwise rails tests runs just fine on Jruby  :+1: 
2. ar - still lots of failing tests
3. railties - still red

#11700