This Saturday (27th, December), I upgraded my project Rails version from version 4.1.5 to version 4.2.0. And I also upgraded the Ruby version from 2.1.2 to 2.1.5.
Today,
I had the same issue and solved it by deleting the Gemfile.lock and running another bundle command.
Gemfile.lock
bundle
After that, I was back to an earlier version of i18n (0.6.11) and everything worked as expected.