Added RS256 alg support via Okta lib

[thomasyip]

- Summary This PR adds support for OAuth2 RS256 alg via Okta's golang lib. There were a few typical pitfalls on implementing OAuth, we feel it was better to use the lib developed by our vendor. (eg, https://github.com/dgrijalva/jwt-go/issues/147)

The PR abstracts auth.go to make the new implementation work alongside with the existing alg. It also makes adding other alg or implementing it with other lib easier later.

We can back-out the Okta implementation upon request. But, we like the abstraction on auth.go to stay such to make it easier for us to keep it in-sync.

- Test plan The Docker on master branch failed to build when we pulled** and had no instruction on how to get it runs. We got it to built and provided some instructions.

** (I later found out there is travis setup. I didn't aware of Travis-ci setup in the beginning.)

We didn't add test in this PR, but believe we improves the quality a little bit over existing condition for the Docker use-case.

The PR should pass Travis-CI. I added an workaround to make Travis-ci works for a fork.

- Description for the changelog Added the option to authenticate using OKTA lib (which use RS256 alg).

• Updated golang to 1.11 (the original docker was on 1.8 and code didn't build on it)
• Added instructions to run git-gateway to README.md
• Added abstraction to auth.go such that different implementations of auth can be used
• Updated example.env
• Updated travis

- A picture of a cute animal (not mandatory but encouraged)

[thomasyip]

@calavera @Benaiah or @rybit is this PR something you guys might consider adopting? I have some time today and tomorrow to address comment. After tomorrow, it will be harder for me to spend time on it. Thank you very much for your time.