Fill out `description` field in Cabal description

This is needed to pass `cabal check`.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Add `cabal {check,sdist}` steps to `.travis.yml`

...and simplify `$PATH` handling while at it. Also, make the build-dep
install workaround for GHCVER=6.12.3 more explicit.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>