I18n.enforce_available_locales should be true.

This gets rid of some deprecation warnings on tests
and in apps using I18n.

ref:
https://github.com/svenfuchs/i18n/commit/3b6e56e06fd70f6e4507996b017238505e66608c

ref:
http://stackoverflow.com/questions/20361428/rails-i18n-validation-deprecation-warning