run apt-get update only once a week

[baranga]

As I provision often while testing my puppet stuff I try to avoid any overhead. Should be enough to update apt-get cache only once a week.

[purple52]

Thanks for the pull request. I see the problem, but I think a simpler solution would be to make apt-get update only run if the script is going to try to install a package. It could simply be wrapped with a check for $FOUND_GIT or $FOUND_LP being non-zero.