Choosing to install SDL2 from a PPA for Travis, as this avoids the use of a Git submodule.
Wasn't really sure how confirmations would be handled by Travis. Turns out they aren't handled at all, so we need to confirm using a pipe.
Should be able to locate packages this way. Hopefully PySDL2 will search in the correct locations after install, will have to see.
Merge branch 'included_dlls' into develop

Not actually including DLLs, as per the original plan, because that would
imply quite a bit of overhead. Will probably do so for production releases,
but for development it's more convenient to use a PPA. Merging this back
into develop because many, or even most future features will interact with
SDL in some way, so that dependency should be taken care of automatically.