chore(saucelabs): fallback from Firefox dev to beta

Closes angular#4553

Author: marclaval

sauce.conf.js

@@ -133,7 +133,7 @@ var aliases = {
   'DEV': ['SL_CHROMEDEV', 'SL_FIREFOXDEV'],
   'CI': ['SL_CHROME', 'SL_ANDROID5.1', 'SL_SAFARI7', 'SL_SAFARI8', 'SL_SAFARI8.1', 'SL_IOS7', 'SL_IOS8', 'SL_IOS9',
          'SL_FIREFOX', 'SL_IE9', 'SL_IE10', 'SL_IE11', 'SL_ANDROID4.1', 'SL_ANDROID4.2', 'SL_ANDROID4.3', 'SL_ANDROID4.4',
-         'SL_CHROMEDEV', 'SL_FIREFOXDEV']
+         'SL_CHROMEDEV', 'SL_FIREFOXBETA']
 };
 
 module.exports = {