If you've installed vagrant-cachier, then use it

You can install vagrant-cachier with vagrant plugin install vagrant-cachier.
If it's not installed nothing changes.
If it is installed the pip, rubygems and apt updates will be cached, making startup a bit faster.

This will break on older versions of Vagrant (prior to 1.3 I think) due to the new nature of Vagrant.has_plugin.
If you got a windows machine you should be up to date, but those people using older machines may need to upgrade.