add optional support for user installed rbenv

this is a followup to the ubuntu 14.04 upstart issue documented here: http://stackoverflow.com/questions/26439555/did-upstart-or-bash-scripts-change-on-ubuntu-14-04-trying-to-boot-sidekiq-with/26439998?noredirect=1#comment41552314_26439998

This adds support for people who installed rbenv in the deploy user directory