test(travis): added travis_retry for testonsauce.sh

[elgalu]

travis_retry checks the return code of a command, in this case testonsauce.sh, retrying up to 3 times if the return code is non-zero.

This will help avoid false positives errored builds due to unrelated timing issues with Sauce Labs.

[juliemr]

didn't know about that, very cool! I'm concerned that this would make the suite take a very long time for legitimate failures, however. (It's already 10 minutes, I'd hate to have it take half an hour).

I'll be exploring some of the saucelabs issues this week - give me a bit to look into that and see if there's other ways to address this before we resort to a full retry :) I'll leave this open though.

[juliemr]

Now that IE 11 is added and I think we've addressed some of the flakes, I'd prefer to find flakes rather than mask them, so I'm gonna close this one.

Thanks for teaching me about travis_retry though!