workaround a bundler crash during 'bundle install'

on ubuntu 12.04 and its native ruby-1.9.3 package, 'bundle install' leads to an encoding error. the fix suggested at http://stackoverflow.com/a/13650994 works.
add stackato.yml (works partially)

deploys fine until db:migrate, https://github.com/discourse/discourse/issues/16
stackato: cache staging assets for faster deployments