Update babel-core and babel-polyfill to 6.7.4

https://github.com/babel/babel/releases/tag/v6.7.4

Author: ctumolosus

package.json

@@ -38,12 +38,12 @@
   ],
   "devDependencies": {
     "babel-cli": "^6.1.2",
-    "babel-core": "^6.2.1",
+    "babel-core": "^6.7.4",
     "babel-eslint": "^4.1.6",
     "babel-loader": "^6.2.0",
     "babel-plugin-transform-es3-member-expression-literals": "^6.5.0",
     "babel-plugin-transform-es3-property-literals": "^6.5.0",
-    "babel-polyfill": "^6.3.14",
+    "babel-polyfill": "^6.7.4",
     "babel-preset-es2015": "^6.3.13",
     "babel-preset-react": "^6.5.0",
     "babel-preset-stage-1": "^6.3.13",