fix(package): update react-dom to version 16.12.0

Closes #38

Author: greenkeeper[bot]

package.json

@@ -8,7 +8,7 @@
   },
   "dependencies": {
     "react": "^16.12.0",
-    "react-dom": "^15.4.0",
+    "react-dom": "^16.12.0",
     "react-router": "^4.0.0",
     "react-router-dom": "^4.0.0"
   },