Upgrade oauth2 from 1.0.0 to 1.1.0

1.1.0 loosens the requirements so Rack 2 can be used with the gem. Rack 2 will be a requirement for Rails 5.

Working towards #14286 (moved).

No Changelog for this gem :/ I guess the commit will do: https://github.com/intridea/oauth2/commit/43e1fdd87e95a5b02438d5cce6a67e411afb5fec