Update Heroku deploy instructions for Swagger-compatibilty and DRYness

I was running into issues with Swagger not showing any docs when deployed
on Heroku and @casualjim helped me by telling me to use ScalatraListener
instead of hooking up Servlets manually in JettyLauncher.

This solved the problem of empty API-docs for me and also seems DRYer (no
more repeated mounting in JettyLauncher and ScalatraBootstrap)
Merge pull request #65 from lloydmeta/update/heroku_deploy_guide

Update Heroku deploy instructions for Swagger-compatibilty and DRYness