chore(travis): fix failures for g3stable/g3v1x. bail early.

- bail early when we can.
- g3v1x requires a newer version of Dart than 1.5.8.  Quickly mark the
  job as successful if we're testing something that's descended from the
  g3v1x branch and we're attempting to use the SDK at this version.
- the g3stable jobs do not make sense if one is testing a branch
  descended from g3v1x.  Bail early with success for those jobs.
chore(travis): download the dart zips in parallel