Install required_packages before 'apt-get update'

This is necessary when required_packages contains GPG keys that are used for
authenticating other packages. Tested with package ubuntu-cloud-keyring which
is included in Ubuntu main and used by the Ubuntu Cloud Archive.

I think the same problem applies to other *-keyring packages as well.
Merge pull request #123 from ctrlaltdel/fix-required-packages

Install required_packages before 'apt-get update'