Karma: use PhantomJS instead of Chrome

Chrome cannot be specified as a dependency and installed automatically,
but PhantomJS can. Most Linux distros use Chromium instead of Chrome, which Karma
does not recognize.

karma.conf.js

@@ -63,7 +63,7 @@ module.exports = function(config) {
         // start these browsers
         // available browser launchers: https://npmjs.org/browse/keyword/karma-launcher
         //browsers: ['Chrome', 'Firefox', 'PhantomJS'],
-        browsers: ['Chrome'],
+        browsers: ['PhantomJS'],
 
 
         // Continuous Integration mode