[#3] Fix guide styling

**Trello**: https://trello.com/c/K8Llw7c6/3-fix-broken-styling-for-guides
**Staging**: http://hnlanswers-stage.herokuapp.com/guides/sample-guide#

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Application.js was loading all of the scripts in app/assets/javascripts.

This included the Markdown scripts which were only used for the CMS and already required explicitly in active_admin.js

Removing the require_tree call from `application.js` fixed the problem.