chore(karma.conf): add support for Firefox
chore(karma.conf): add support for Firefox

Add `karma-firefox-launcher` to `karma.conf.js`. Note that it is
already included in `package.json`.

Closes #815

trinarytree/angular.dart,SibuStephen/angular.dart,winstonewert/angular.dart,rkirov/angular.dart,trinarytree/angular.dart,rkirov/angular.dart,hitesh97/angular.dart,hitesh97/angular.dart,winstonewert/angular.dart,giovannicandido/angular.dart,giovannicandido/angular.dart,ronlobo/angular.dart,hitesh97/angular.dart,rkirov/angular.dart,hitesh97/angular.dart,ronlobo/angular.dart,winstonewert/angular.dart,trinarytree/angular.dart,trinarytree/angular.dart,giovannicandido/angular.dart,SibuStephen/angular.dart,SibuStephen/angular.dart,SibuStephen/angular.dart,ronlobo/angular.dart,winstonewert/angular.dart,rkirov/angular.dart,ronlobo/angular.dart