Update travis.yml to find missing libsdl2-dev package
Update travis.yml to find missing libsdl2-dev package

Travis uses ubuntu 12.04 LTS, which does not have the package
libsdl2-dev by default. This commit provides uses a ppa to install it
instead.

We cannot yet use Travis' apt extension (which does contain libsdl2-dev),
since libdirectfb-dev is not present. A request has been made here:
https://github.com/travis-ci/apt-package-whitelist/issues/2140

Tiehuis/mptet