Update to Rails 4.

Also update other gems including:
thin, pg and omniauth-google-oauth2.
Use get instead of match.
Remove superfluous translation YAML keys.
Create helper method to prettify provider name.
Let OmniAuth keep track of the HTTP Referer for us.

This is will lead to a simpler callback since we can be confident that
env['omniauth.origin'] will always contain the url the user tried to
access.

https://github.com/intridea/omniauth/wiki/Saving-User-Location
Properly preserve the environment on failure.
Remove code duplication!