Use "apt-get" in cron script

[dongbohu]

I just realized that using apt command in cronjob script is not as good as using apt-get (whose command line interface is more stable), so I am changing apt into apt-get in this script.

[dongbohu]

@dhimmel

[dhimmel]

@dongbohu looks like you started this branch from an outdated commit-- notice the long list of commits. This is not an issue here because I will squash merge, but should avoid this in future.