Extract Grape::Middleware::Auth::OAuth2, which was deleted in grape 0.9

According to https://github.com/intridea/grape/blob/master/UPGRADING.md

Grape::Middleware::Auth::OAuth2 have been removed and current grape-doorkeeper version does not compatible with newest Grape 0.9 version.

Just extract Grape::Middleware::Auth::OAuth2

And fix gemspec file