chore(package): update react-dom to version 16.4.2

Closes #6

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
     "jest": "^21.0.0",
     "koa": "^2.3.0",
     "react": "^16.4.2",
-    "react-dom": "^15.6.1",
+    "react-dom": "^16.4.2",
     "webpack": "^3.5.4"
   },
   "dependencies": {