Keep 'major.minor' version boxes for all distributions

[blbradley]

I have the need to test against Debian 7.0 and wondering if boxes that are not the latest version of a distro will be dropped when the boxes are regenerated.

Ex: Debian 7.2 comes out and this repo is updated. Generated 7.0, 7.1, and 7.2 instead of just one. Same for other distros.

Linode is an example one host that runs an old minor version.