Update to latest Elixir and OAuth2, fix issue with get_token!

[vinniefranco]

• Updates to latest version of Ueberauth
• Changes things a bit to go off latest github example
• Updates get_token! to explicitly pass client_id and client_secret which fixes #1
• Verified with latest example app

[vinnie-eb]

I verified this was working with another app - thanks for the fixes! I do have some general questions about the approach - particularly around why the variables need to be manually merged in (I'm looking at the source for the ueberauth_facebook project and noticing that they don't manually merge the client_id param on their side

I noticed that as well. This PR updates OAuth2 to the same release as the FB strategy that you provided... so not too sure there. I just know that it did not work until I explicitly provided client_secret, and client_id

[andrewhao]

@vinnie-eb - your work here makes sense. I was comparing it to the code in the ueberauth_facebook repo, which takes yet a different approach. In any case, I'll add some tests, which is the biggest missing piece, anyways.