Add support for OmniAuth 2.0

[shmokmt]

I think it is safer to support OmniAuth2.0 for this gem as well like omniauth/omniauth-openid#44 .

See Also:
https://github.com/omniauth/omniauth/releases
heartcombo/devise#5326 (comment)

[travisofthenorth]

@m0n9oose do you mind taking a look at these and related PRs? It would be great to get this gem upgraded so that applications using it can upgrade omniauth and address related security issues.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.