fix(package): update react-dom to version 16.8.1

Closes #30

Author: greenkeeper[bot]

package.json

@@ -8,7 +8,7 @@
   "private": true,
   "dependencies": {
     "react": "^16.8.1",
-    "react-dom": "^15.4.2"
+    "react-dom": "^16.8.1"
   },
   "devDependencies": {
     "@types/enzyme": "^2.7.6",