fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-SHOWDOWN-469487

package.json

@@ -37,7 +37,7 @@
     "quill": "2.0.0-dev.2",
     "qz-tray": "^2.0.8",
     "redis": "^2.8.0",
-    "showdown": "^1.8.6",
+    "showdown": "^1.9.1",
     "socket.io": "^2.0.4",
     "superagent": "^3.8.2",
     "touch": "^3.1.0",