Upgrade Bundler on Travis if it's old or unstable
Upgrade Bundler on Travis if it's old or unstable

(I'm seeing a worker with a copy of 1.3.0pre8 experience test failures
because it prints some warnings that were fixed by 1.3.0 final:
https://travis-ci.org/chrisk/fakeweb/jobs/7085551)

chrisk/fakeweb