.travis.yml: Upgrade gdb and use python-dbg so cygdb tests can run

The Travis CI build was skipping the cygdb tests because the default gdb installed wasn't >= 7.4 and because we weren't using a debug build of python (i.e.: `python-dbg`).

This PR fixes both of those issues so that Travis CI can run the cygdb tests (which incidentally, are failing, even though they're not properly returning a failed exit status so that the Travis CI build is marked as a failure).

See this Travis CI job which shows that with this PR it's now running the cygdb tests (and failing):

    https://travis-ci.org/msabramo/cython#L366