Use the CommonJS entry point for fetch-mock

The entry point for fetch-mock v8 is an ES module which Browserify does
not support natively. Therefore use the CommonJS entry point as
described at http://www.wheresrhys.co.uk/fetch-mock/#usageimporting.

package.json

@@ -136,6 +136,7 @@
   },
   "browser": {
     "hammerjs": "./node_modules/hammerjs/hammer.js",
+    "fetch-mock": "./node_modules/fetch-mock/cjs/client.js",
     "jquery": "./node_modules/jquery/dist/jquery.slim.js"
   },
   "browserify-shim": {