fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-INFLIGHT-6095116
- https://snyk.io/vuln/SNYK-JS-MINIMATCH-1019388
- https://snyk.io/vuln/SNYK-JS-MINIMATCH-3050818
- https://snyk.io/vuln/SNYK-JS-NPMUSERVALIDATE-1019352
- https://snyk.io/vuln/npm:minimatch:20160620
- https://snyk.io/vuln/npm:npm:20160418

package.json

@@ -7,7 +7,7 @@
     "test": "test"
   },
   "dependencies": {
-    "gitbook-cli": "~2.2.0",
+    "gitbook-cli": "~2.3.1",
     "gitbook-plugin-edit-link": "~2.0.2",
     "gitbook-plugin-github": "~2.0.0"
   },