Update to omniauth 2

I've included the changes from https://github.com/omniauth/omniauth_openid_connect/commit/e87a906a5c702081f87fad8516e724cccdcd2b9d to update the gem to omniauth 2.0 since this gem hasn't been updated for long. I'm working on omniauth 2.0 migration in Debian and this gem is blocker for us. Please take a look and merge or consider updating the gem. This will make things easier for us :)

Cheers