Travis: Cope with ftpmirror.gnu.org being down

Sometimes builds fail because ftpmirror.gnu.org is down.  With this change
Travis will fall-back to downloading GNU parallel from main GNU FTP site
instead if the mirrors fail.  Failing that we don't bother with GNU
parallel at all and gracefully degrade to running the tests in series.
Merge pull request #231 from wmanley/parallel-failure-robustness

Travis: Cope with ftpmirror.gnu.org being down