chore(package): update web-ext to version 2.2.1

Closes #101

Author: greenkeeper[bot]

package.json

@@ -33,7 +33,7 @@
     "npm-run-all": "4.1.1",
     "selenium-webdriver": "3.5.0",
     "virtualenv": "0.3.1",
-    "web-ext": "2.0.0"
+    "web-ext": "2.2.1"
   },
   "permissions": {
     "multiprocess": true