Fixes #3480 Show how to install and use html-proofer without Gemfile

site/_docs/continuous-integration.md

@@ -118,6 +118,7 @@ customizable. If your script won't change much, you can write your test
 incantation here directly:
 
 {% highlight yaml %}
+install: gem install jekyll html-proofer
 script: jekyll build && htmlproof ./_site
 {% endhighlight %}