chore(package): update @babel/core to version 7.4.0

Closes #235

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
     "magic-string": "0.25.2"
   },
   "devDependencies": {
-    "@babel/core": "7.3.4",
+    "@babel/core": "7.4.0",
     "@babel/preset-env": "7.3.4",
     "del": "4.0.0",
     "eslint": "5.15.3",