Travis to build gh-pages + zip for release

[klokan]

Add travis to build the gh-pages and force push these.

The script for building will call the generate.sh.

We have to add also release handle - which will create a .zip in release. Such zip does not need to contained individual .zips for each font which are pushed to gh-pages for downloading from the mobile apps.

https://docs.travis-ci.com/user/deployment/releases/

[klokan]

I have made the release manually now.

It has an attached file with generated .pbf's for all fonts: https://github.com/openmaptiles/fonts/releases/download/v1.0/v1.0.zip

This file is going to be used in the deploy scripts, instead of gh-pages branch cloning