Add support for adding cron jobs through Homestead.yml

From https://github.com/laravel/homestead/pull/89

>Make it possible to define crontabs and getting them installed for 'root' on VM: (minute/hour/monthday/week/weekday defaults to '*' if left out)

>```
crontabs:
    - command: /home/vagrant/Code/jobs/addresses-import-job.sh
      minute: '*/15'
      hour: '*'
      monthday: '*'
      week: '*'
      weekday: '*'
```