fix: package.json & yarn.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-SERIALIZEJAVASCRIPT-536840

Author: snyk-bot

package.json

@@ -16,7 +16,7 @@
     "react-markdown": "^4.0.8",
     "react-router": "^4.3.1",
     "react-router-dom": "^4.3.1",
-    "react-scripts": "^3.0.1",
+    "react-scripts": "^3.3.0",
     "styled-components": "^4.3.1"
   },
   "scripts": {