fix: upgrade multiple dependencies with Snyk

Snyk has created this PR to upgrade:
  - react from 16.4.1 to 16.13.0.
    See this package in NPM: https://www.npmjs.com/package/react
  - react-dom from 16.4.1 to 16.13.0.
    See this package in NPM: https://www.npmjs.com/package/react-dom

See this project in Snyk:
https://app.snyk.io/org/yowainwright/project/03b099ed-f55b-4b6c-983e-577944d94ddf?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

package-lock.json

@@ -48,10 +48,10 @@
     "open-browser-webpack-plugin": "^0.0.5",
     "prop-types": "^15.5.10",
     "purecss": "0.6.0",
-    "react": "^16.4.1",
+    "react": "^16.13.0",
     "react-addons-css-transition-group": "^15.1.0",
     "react-addons-transition-group": "^15.1.0",
-    "react-dom": "^16.4.1",
+    "react-dom": "^16.13.0",
     "react-motion": "^0.4.2",
     "react-redux": "^5.0.2",
     "react-router-dom": "^4.0.0",