Update handlebar to version 4.0.13

Synk found an security vulnerbility in the version we provide, that in
theory can provide an RCE.

Details: https://snyk.io/vuln/SNYK-JS-HANDLEBARS-173692

Author: SISheogorath

package.json

@@ -46,7 +46,7 @@
     "formidable": "^1.0.17",
     "gist-embed": "~2.6.0",
     "graceful-fs": "^4.1.11",
-    "handlebars": "^4.0.6",
+    "handlebars": "^4.0.13",
     "helmet": "^3.13.0",
     "highlight.js": "~9.12.0",
     "i18n": "^0.8.3",