Page fails to load on old Firefox versions

[andrenarchy]

The page fails to load with older Firefox versions (verified with 38 and 42). Since the execution of the js bundle is stopped, the page remains empty. The displayed error message is

TypeError: Function.prototype.toString called on incompatible object 

The error is caused by async, see caolan/async#1158.

Thanks to Michael Rothgang for reporting the issue!

[andrenarchy]

I also included Firefox 38 in the saucelabs tests (fails as expected): https://saucelabs.com/beta/tests/fe5d5de5628c4a9ba25b7e33c1de2c47/commands