Merge pull request circleci#2669 from circleci/add-crystal-to-demo-apps-page

Update demo-apps.md

Author: michelle-luna

jekyll/_cci2/demo-apps.md

@@ -13,6 +13,7 @@ Language Guide | Framework | GitHub Repo Name
  ---------|-----------|-----------------
  [Android](https://github.com/CircleCI-Public/circleci-demo-react-native/blob/master/README.md){:target="_blank"} | React Native | [circleci-demo-react-native]{:target="_blank"}
  [Clojure]{:target="_blank"} | Luminus | [circleci-demo-clojure-luminus]{:target="_blank"}
+ [Crystal]({{ site.baseurl }}/2.0/language-crystal/) | Kemal | [circleci-demo-crystal](https://github.com/teesloane/circleci-demo-crystal)
  [Elixir]{:target="_blank"} | Phoenix | [circleci-demo-elixir-phoenix]{:target="_blank"}
  [Go]{:target="_blank"} | Go | [circleci-demo-go]{:target="_blank"}
  [iOS]{:target="_blank"} | Xcode | [circleci-demo-ios]{:target="_blank"}