[DRE-maint] Bug#1031441: Relax oauth2 dependency

[Daniel Leidert]

I just rebuilt ruby-github-api by relaxing the dependency on oauth2. I
pulled the test cases from github to check if anything happens. It
built just fine. 6 tests failed, but none related to OAuth2 (seems some
keywords/options mismatch).

I think we could fix this by relaxing the dep on oauth2 for now.

There is an upstream report asking for supporting oauth2 2.x at
https://github.com/piotrmurach/github/issues/386.

Regards, Daniel