Add rbenv rake

[adelevie]

Shamelessly copied the solution described here http://benscheirman.com/2013/12/using-rbenv-in-cron-jobs/.

For rake tasks, just use rbenv_rake "my:rake:task" instead of rake "my:rake:task.

[adelevie]

I think I want to close my own PR. The code here is OSX specific. FWIW, here's how I got this working on Ubuntu:

job_type :rbenv_rake, %Q{export PATH=/home/alan/.rbenv/shims:/home/alan/.rbenv/bin:/usr/bin:$PATH; eval "$(rbenv init -)"; \
                         cd :path && bundle exec rake :task --silent :output }