fix: Gemfile to reduce vulnerabilities (#137)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-COMMONMARKER-3318398
- https://snyk.io/vuln/SNYK-RUBY-COMMONMARKER-3318399
- https://snyk.io/vuln/SNYK-RUBY-COMMONMARKER-3318400
- https://snyk.io/vuln/SNYK-RUBY-COMMONMARKER-3318401

Gemfile

@@ -15,7 +15,7 @@ gem "minima", "~> 2.5", ">= 2.5.1"
 
 # If you want to use GitHub Pages, remove the "gem "jekyll"" above and
 # uncomment the line below. To upgrade, run `bundle update github-pages`.
-gem "github-pages", "~> 219", group: :jekyll_plugins
+gem "github-pages", "~> 224", group: :jekyll_plugins
 
 # If you have any plugins, put them here!
 group :jekyll_plugins do