refactor(CI): use ChromeHeadless launcher

Author: jshcrowthe

gulp/config.js

@@ -103,7 +103,7 @@ module.exports = {
 
     // start these browsers
     // available browser launchers: https://npmjs.org/browse/keyword/karma-launcher
-    browsers: ['Chrome', 'Firefox'],
+    browsers: ['ChromeHeadless', 'Firefox'],
 
     // Continuous Integration mode
     // if true, Karma captures browsers, runs the tests and exits