Merge pull request #1755 from mozilla/greenkeeper/fetch-mock-5.9.3

chore(package): update fetch-mock to version 5.9.3

package.json

@@ -222,7 +222,7 @@
     "eslint-plugin-import": "2.2.0",
     "eslint-plugin-jsx-a11y": "2.2.3",
     "eslint-plugin-react": "6.8.0",
-    "fetch-mock": "5.8.1",
+    "fetch-mock": "5.9.3",
     "file-loader": "0.10.0",
     "glob": "7.1.1",
     "json-loader": "0.5.4",