test($ci): #1089 - Remove SauceLabs, Integrate SlimerJS
test($ci): #1089 - Use headless Firefox rather than SlimerJS, for now.

Until SlimerJS becomes more stable (1.0?), going to use headless Firefox.
Karma version is bumped in this commit too.
Merge branch 'feature/ditch-sauce' into develop

* feature/ditch-sauce:
  test($ci): #1089 - Use headless Firefox rather than SlimerJS, for now.
  test($ci): #1089 - Remove SauceLabs, Integrate SlimerJS