Fix image name (12.04.2 seems to be missing).

Not sure the exact semantics of these paths, but 12.04.2 was not available.
This was actually a page pointing to the base 12.04 image, which I think is the
current link to use here.

Signed-off-by: Adin Scannell <adin@scannell.ca>
Merge pull request #53 from amscanne/fix_updating

Fix image name (12.04.2 seems to be missing).