Add upgradevm command

[ocdtrekkie]

Fixes #245

Super simple for now. It grabs the same code as setupvm but just for Vagrantfile and global-setup.sh, which shouldn't generally be modified by app packagers.

As a test, I grabbed the GitWeb source prior to @abliss updates, because I knew he hit the 404 box issue. Went through the flow and failed to start the VM. I ran vagrant-spk upgradevm and then I was able to start up the VM. (Although I also got a 502 Bad Gateway because there was other related bitrot for GitWeb.)

Note that currently neither setupvm nor upgradevm has any sort of protection to prevent you from clobbering what you have. (This is worse in setupvm's case, because it can mess up your app build.) Though hopefully people are committing to GitHub or have original sources to fall back on. But we can do better here. We should also put comments at the top of both files we upgrade this way to further discourage user customization to make this command 'safer'.

Also, I'm not yet detecting the box that caused GitWeb to 404, and we're going to amend the detection for that to do so.

[ocdtrekkie]

This now also fixes #244

There was a lot of complicated logic in deciding not to flip out if someone pinned an old enough jessie64 box. I decided we don't want to continue to support super old pinned versions of jessie64 boxes anyways, and scrapped all of it. Though I left instructions to update to a supportable contrib-jessie64 box if someone really wants to get a package working on Jessie, we'd prefer people upgradevm to clear out any and all Vagrantfile-related bitrot, of which there is plenty since that era of vagrant-spk.

[ocdtrekkie]

Now when downloading the pre-Adam-updated GitWeb, instead of getting a 404 for trying to download the box that doesn't exist, we tell people they should upgradevm or switch boxes to contrib-jessie64. After running upgradevm, the correct versions are placed in the directory and the VM can correctly boot up.

[abliss]

Cool!

[zenhack]

Some minor bitrot re: mention of distro name, but otherwise LGTM. We should proabably add an 'are you sure?' or something at some point, but I'm fine with it as is for a first pass.