Add SDL2 to the INSTALL file

Tested on debian and fedora systems, the instructions for arch and windows
are untouched.
adapt .travis.yml continuous build integration for sdl2
Revert "adapt .travis.yml continuous build integration for sdl2"

travis doesn't have sdl2 yet apparently.

This reverts commit 99544132666572e90aa4b4e09e3ad7becf685d1f.
Run travis with sdl 1.2
Merge pull request #1094 from stefanbeller/readme

Add SDL2 to the INSTALL file