fix(package): update react-dom to version 16.9.0

Closes #11

Author: greenkeeper[bot]

package.json

@@ -13,7 +13,7 @@
   "dependencies": {
     "http-server": "^0.11.1",
     "react": "^16.9.0",
-    "react-dom": "^15.4.1",
+    "react-dom": "^16.9.0",
     "steal": "^1.0.10",
     "steal-jsx": "^0.0.2"
   },