Update Travis script for SDL2

I've updated the Travis CI script to build the SDL2 version.

Currently it'll fail because of the lack of LevelEdit ant build file which was included in the corresponding pull request on the master branch. When this branch is brought back up to speed with the master branch then this should work, so for now please disregard the nasty warning that'll appear on this pull request!

I've had to make a minor change to a couple of .cpp files, to include *string.h*. It wouldn't build without this. I'd appreciate it if someone could test to make sure that this doesn't break building on other platforms.

Although it uses mingw-w64, it still only builds Win32 binaries. It'd only require a small change to build 64-bit ones too, but I don't see much value in doing this right now.