bundle exec not working with crontab

The user used by cron does not have the correct environment. You can tell cron which user to use. For a bash script, you can so something like:

#!/bin/bash --login
source /home/user/.bashrc
rvm use 2.0.0@gemset #if you use rvm
cd /path/to/project && bundle exec xyz

A nice trick to get all environment properly set up in crontab is to use /bin/bash -l :

0 16 * * * /bin/bash -l -c '/mnt/voylla-production/releases/20131031003111/voylla_scripts/cj_4pm.sh'

The -l option will invoke a full login shell, thus reading your bashrc file and any path / rvm setting it performs.

If you want to simplify your crontab management and use this trick - as well as others - without having to think about them, you can use the Whenever gem. It also play very nice with capistrano, if you use it, regenerating crontab on deploy.