Upgrade oauth2 from 1.0.0 to 1.2.0.

Changelog: https://github.com/intridea/oauth2/compare/v1.0.0...v1.2.0

Follow-up on !3434 (closed) since 1.2.0 doesn't limit the JWT version we can use.

Adds support for Rails 5. Working toward #14286 (moved).

cc: @rspeicher