nailgun 0.9.1 (zombie formula)

As per #21880 nailgun was removed because it was woefully outdated and had moved to a new packaging format.
This commit brings the nailgun formula back from the boneyard and updates it to 0.9.1.
nailgun 0.9.1 (fixed comment)

Needed a few more words.
nailgun 0.9.1 (use a thread)

@mikemcquaid's feedback
nailgun: add 0.9.1 bottle.