Using master branch leads to Javascript TypeError

[Whitie]

The commit 17ac00d introduced a static method to retrieve the bar colors. Calling a static method from the constructor fails with a TypeError. See https://stackoverflow.com/questions/37002804/calling-static-method-from-constructor-es6 for explanation.
Line 25 of celery_progress.js must be changed from:

this.barColors = Object.assign({}, getBarColorsDefault(), options.barColors);

to:

this.barColors = Object.assign({}, this.constructor.getBarColorsDefault(), options.barColors);

It's just a little change, should I make a pull request for this?

Whitie

[czue]

Eep, sorry about that! A PR would be great!