Add Travis support & some packaging cleanups

I took the liberty of modernizing `.cabal` to simplify testing with Travis; if there's something you don't like about this, please lemme know;

after you merge this, you still need to  actually activate the Travis job by going to travis-CI and following the instructions (if you need more directions, lemme know)

If this works out, I'll prepare a pull-request for `haskeline` as well.