fix sounds not playing on first play (=load), because function bailed on success, rather than error. introduced in bd9a2f1b0080 , addressing
issue #111
travis: Use correct vorbis flag in sdl2_mixer install
Revert the addition of cmake which is already instealled.
Hijack SDL2 and GLEW include and library when compiling to javascript
emscripten already provides the SDL2 suite and will simply ignore
missing libraries.