fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-IP-6240864

Author: snyk-bot

package.json

@@ -6,7 +6,7 @@
   "dependencies": {
     "core-js": "^2.6.0",
     "disqus-react": "^1.0.4",
-    "gatsby": "^2.0.0",
+    "gatsby": "^3.13.0",
     "gatsby-image": "^2.0.22",
     "gatsby-link": "^2.0.7",
     "gatsby-plugin-canonical-urls": "^2.0.8",