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 (https://github.com/scalatra/scalatra/issues/290#issuecomment-35199221) 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)