See #4462 for the gory details.

Main things about this commit:

- New 'monolithic' flag on cabal-install, which combines
  all of the tests into a single binary.  It's not very
  much code, and you don't pay for any of it on a release
  build.  I quite like it.  The one downside is that
  we can't also pull in Cabal test suites this way.

- Env vars got moved into travis-common.sh

- travis-script.sh now runs the cabal-tests tests, because
  we aren't sending enough build product over to do them
  on the second Travis run

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>