Added version check when syncing repo (requires portmod_vx.x tags)

This won't actually show up until we start making releases, however it provides a way of informing users if their portmod version is out of date compared to the one in use when the openmw-mods repo was last modified.

This will also require maintaining the version tags on HEAD, but we can probably use a hook to have a machine automatically update the tag for the latest version of portmod whenever changes are merged to openmw-mods.

Closes #58 (closed).