Lock to Firefox 54 until Travis CI is fixed

[etimberg]

Try and get the tests to pass by forcing Travis to use Firefox 54 instead of the latest (56).

The tests seem to be failing with an error:

07 10 2017 13:55:53.062:WARN [Firefox 56.0.0 (Linux 0.0.0)]: Disconnected (1 times), because no message in 10000 ms.
  Disconnected, because no message in 10000 ms.
Firefox 56.0.0 (Linux 0.0.0) ERROR

These tests pass fine locally with Firefox 56. This is likely related to travis-ci/travis-ci#8242

[simonbrunel]

Per this comment, locking FF version doesn't work anymore (confirmed by this PR Travis build) :\

[etimberg]

Another option is to remove Firefox and only test chrome. Not sure how good of an idea that is

[simonbrunel]

The last merge didn't fail, not sure if it's a coincidence or if it fixes the Firefox issue.

[etimberg]

hmmm, maybe it's related

[benmccann]

I just sent a new PR and the build passed on it as well: #4826

I'm guessing the last merge may actually have solved the issue

[etimberg]

Cool. I'll close this out then since it't not needed