Major overhaul of the rbenv role

 * Follow rbenv installation procedure more closely
 * Don't set global ruby by default
 * Cleaner handling of env variables
 * Extract ruby_version to be specified by the caller
Rework nginx-passenger role for new rbenv config
Change Tracks playbook to work with new rbenv config
Include screen in standard server packages
Update Tracks code
Better method for setting rbenv version
Add missing rake tasks
Bump Tracks version