Update Google authentication provider to latest `easy_auth-oauth2`

This is very similar to dockyard/easy_auth-oauth2#5

I also updated the gems and renamed the test file to `features` directory as capybara wants it.

Off-topic:
I just saw that `easy_auth-oauth2` pull request is still open and no comments were left. Is there something I can help with to get it merged or update the gem?

Thanks.