If you use whenever and find bash -l -c won't load rbenv, here's a hacky way to fix it.

rbenv_bash

#!/bin/bash
# Save this file to /usr/bin/rbenv.bash, chmod +x it, and set SHELL variable in crontab to this.

export RBENV_ROOT=/usr/local/rbenv
export PATH="$RBENV_ROOT/bin:$PATH"
eval "$(rbenv init -)";

bash "$@"