Merge pull request #16 from KnisterPeter/greenkeeper-babel-core-6.3.21

Update babel-core to version 6.3.21 🚀

Author: KnisterPeter

package.json

@@ -28,7 +28,7 @@
   },
   "license": "MIT",
   "devDependencies": {
-    "babel-core": "6.3.17",
+    "babel-core": "6.3.21",
     "babel-preset-es2015": "6.3.13",
     "babel-register": "6.3.13",
     "chai": "3.4.1",