Clean up project

* Simplify build process and remove unneeded files.
* Move all files required to build the site into `src/`.
* Use the browser logos from https://github.com/alrra/browser-logos,
  so that they are always up to date.
Make Travis automatically update gh-pages branch

Configure Travis to automatically update the content from the
`gh-pages` branch whenever something is pushed to the `master`
branch and the tests pass.