Add invariant dependencie

Author: NoelBroda

Diff for: package.json

@@ -67,6 +67,7 @@
   },
   "dependencies": {
     "bows": "^1.6.0",
-    "es6-symbol": "^3.1.0"
+    "es6-symbol": "^3.1.0",
+    "invariant": "^2.2.2"
   }
 }