Convert expires_at to integer

We're using the google-oauth2 gem with oauth2 and devise, and recently google have started supplying the expires_at value. Since this is a string, oauth2 explodes when trying to evaluate if a token has exploded (string doesn't implement the < operator).

So like you've done with expires_in, we convert it to an integer. We've written tests and she's all green.

Note that we're using 0.8.0 because of https://github.com/intridea/omniauth-oauth2/issues/20 but this cleanly applies to HEAD as well.