New workflow / master branch changed / Travis CI integrated

[lutzthies]

The default branch has been changed to development.

Travis CI is watching that branch and a new build is triggered after every push. If it succeeded, the generated static files will be automatically force-pushed to gh-pages, so that a preview is displayed via GitHub Pages.

The master branch is used for releases that are automatically deployed to the production site https://ese.ifsr.de/2017/.