chore(package): update babel-core to version 6.9.0 (#111)

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -37,7 +37,7 @@
   },
   "license": "MIT",
   "devDependencies": {
-    "babel-core": "6.8.0",
+    "babel-core": "6.9.0",
     "babel-preset-es2015": "6.9.0",
     "babel-register": "6.9.0",
     "chai": "3.5.0",