fix(package): update react-dom to version 16.8.6

Closes #14

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "babel-preset-react": "^6.5.0",
     "babel-standalone": "^6.4.4",
     "react": "~16.8.6",
-    "react-dom": "^15.4.0"
+    "react-dom": "^16.8.6"
   },
   "devDependencies": {
     "assume": "~1.5.0",