Update Travis with SDL2-2.0.3

See here for a changelog in this new version: http://hg.libsdl.org/SDL/shortlog
Include patch from upstream to fix SDL2 2.0.3 build in mingw: https://hg.libsdl.org/SDL/changeset/e217ed463f25/include/SDL_platform.h
Merge pull request #320 from TheCycoONE/sdl2_2_0_3

Update Travis with SDL2-2.0.3