Add inverse_of to translations association

While working on fixing refinery/refinerycms#2450 I tried out adding an `inverse_of` option to the `translations` association. There were a couple kinks (two failing tests which I fixed) but it seems to work fine, and I think all-in-all it would be good to have this.

@parndt any issues you can imagine from doing this? I've tested on refinery 2-1-stable and see no failures.