New formula: saltstack v0.13.1

Saltstack is a Python-based configuration management tool, similar to Puppet or Chef.

NOTE: this Homebrew formula is based heavily upon work by @singingwolfboy (who created a Homebrew tap for saltstack), but with revisions to conform the formula to Homebrew's current requirements.

See more https://github.com/singingwolfboy/homebrew-saltstack