Add a test for id_was and switch from .rvmrc to .ruby-version style

Pursuant to the conversation here https://github.com/composite-primary-keys/composite_primary_keys/pull/245#issuecomment-58655102 I added a test for `id_was`.

I also got tired of RVM telling me .rvmrc was slow and incompatible with other Ruby version managers, so I did the migration it suggested.

I put these on master to make sure the current versions get the new test, but we should also merge this on  the ar_4.2.x branch.