Add missing dependency.

Author: ghengeveld

package.json

@@ -46,6 +46,7 @@
   },
   "devDependencies": {
     "babel-cli": "6.26.0",
+    "babel-core": "6.26.3",
     "babel-eslint": "10.0.1",
     "babel-preset-env": "1.7.0",
     "babel-preset-react": "6.24.1",