apt::source with required_packages may fail

If apt::source is called from within a manifest including
required_packages installation of packages may fail if local
package lists are out of date (common issue when using
Vagrant with "old" base boxes).
Have apt::source trigger apt::update BEFORE installation of
required_packages fixes that issue for me, even if I do not
fully understand the complete architecture of that module and
I am not aware of possible problems this commit may introduce.