Not compatible with rails 5

[devijonnalagadda]

I used this gem by downloading it from github and placed in the rails app lib folder. I specified the :path in gemfile. Made some changes in the gem source code. Some of the methods were depreciated in rails 5, so i replaced them with the newer one's. This is very useful gem for 2FA. I suggest to don't stop this gem implementation, please continue its implementation. To make it compatible with rails 5, very small changes were needed. And also add backup code implementation.

[Guillaume Hain]

Hey @devijonnalagadda, we are using this gem at Pharmony and we have a working fork in production, have a look at here: https://github.com/Pharmony/devise_google_authenticator/tree/compatibilites/rails5