Add github pages deployment to travis

[zakpatterson]

On successful build from travis, the most recent generated docs should be deployed to the github pages site.

So far this has been done manually: https://github.com/pauljamescleary/scala-pet-store/deployments

Adding docs deployment to the travis configuration

Necessary steps:

1. ⬜️ Add secure github api token to repository so it can be used in travis build
2. ⬜️ Add tut build step to travis build, fail the build if tut fails.
3. ⬜️ Deploy docs to github pages if successful, fail the build if deployment fails.

[pauljamescleary]

I actually have this setup on vinyldns - https://github.com/vinyldns/vinyldns

This is the sbt configuration...
https://github.com/vinyldns/vinyldns/blob/546cd49128db733f0bf795e11de028332b799499/build.sbt#L378

Here is the travis configuration...

https://github.com/vinyldns/vinyldns/blob/546cd49128db733f0bf795e11de028332b799499/.travis.yml#L46

We are using plugins...

addSbtPlugin("com.typesafe.sbt" % "sbt-ghpages" % "0.6.2")

addSbtPlugin("com.47deg"  % "sbt-microsites" % "0.7.24")

[zakpatterson]

Another example, using the deploy: travis task