Merge pull request #44 from wework/greenkeeper/babel-core-6.21.0

chore(package): update babel-core to version 6.21.0

Author: nason

package.json

@@ -56,7 +56,7 @@
     "express": "^4.14.0"
   },
   "devDependencies": {
-    "babel-core": "6.18.2",
+    "babel-core": "6.21.0",
     "babel-loader": "6.2.10",
     "babel-plugin-external-helpers": "6.18.0",
     "babel-plugin-istanbul": "3.0.0",