Require omniauth.rb in strategy.rb

Without this, the usage of `OmniAuth.strategies` on line 12 (previously line 11) sometimes fails with `undefined method 'strategies' for OmniAuth:Module (NoMethodError)` when strategy.rb is required from a dependent gem without omniauth.rb having been loaded.

I encountered this while trying to use the omniauth-cas gem. I was going to submit a patch to that gem, but realized this would potentially affect all gems with omniauth as a dependency, so it makes more sense to do it here.