Require capistrano-rbenv only in staging
Require capistrano-rbenv only in staging

Production is not provisioned with Rbenv yet. We're first refactoring
staging's provisioning and then, we'll work on production. So, this
patch is temporary.

This fixes the issue we had in
https://github.com/coopdevs/timeoverflow/releases/tag/v1.3.0 while
deploying and
https://github.com/coopdevs/timeoverflow/releases/tag/v1.3.1 fixed.

coopdevs/timeoverflow,coopdevs/timeoverflow,coopdevs/timeoverflow,coopdevs/timeoverflow