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

Closes #43

Author: greenkeeper[bot]

package.json

@@ -48,7 +48,7 @@
     "prettyjson": "1.1.3",
     "react": "^16.4.2",
     "react-codemirror": "0.2.6",
-    "react-dom": "^15.3.2",
+    "react-dom": "^16.4.2",
     "react-transform-catch-errors": "1.0.2",
     "react-transform-hmr": "1.0.4",
     "redbox-react": "1.3.2",