Updated SDL libraries fixing OS X fullscreen bug

Updated SDL libraries in OS X build fixing a bug where versions of OS X
after 10.6.8 would crash when trying to run Assault Cube in fullscreen.
SDL libraries are latest for both SDL.framework (1.2.15) and
SDL_image.framework (1.2.12).
Removed references to SVN, improved grammar

As title states. Removed references to SVN and replaced with Git.
Improved grammar in last sentence for increased readability