Merge pull request #1273 from mozilla/greenkeeper-babel-preset-stage-2-6.18.0

Update babel-preset-stage-2 to version 6.18.0 🚀

package.json

@@ -201,7 +201,7 @@
     "babel-plugin-transform-object-rest-spread": "6.16.0",
     "babel-preset-es2015": "6.16.0",
     "babel-preset-react": "6.16.0",
-    "babel-preset-stage-2": "6.17.0",
+    "babel-preset-stage-2": "6.18.0",
     "babel-register": "6.16.3",
     "bundle-loader": "0.5.4",
     "chai": "3.5.0",