vim: roll back and track debian-unstable

Vim sees patchlevel release every few days (and recently, several times
per day). This is too often to be updating the formula and prompting
upgrades for users who don't care to update that frequently. It is also
a vector for bugs when new development is happening upstream (e.g. the
recent regexp engine switch).

So this package now tracks debian-unstable. People who want to live on
the bleeding edge can still use --HEAD like any other formula.