Automate the deployment of the docs pages

[dilyand]

From @peel:

The simplest option is probably to modify .travis/deploy.sh to do sbt doc and commit ./target/scala-2.12/api to gh-pages branch
But travis allows for multiple deploy providers! That would be way easier and we wouldn't need to use the script. We'd just need a doc generation step and then we can use the regular deploy section with provider: pages