Add a sandboxed version of Homebrew's `Utils.popen_read`
Add a fallback to `OS::Mac`

Currently, Homebrew still provides `MacOS`, but it is already
deprecated.
Skip validation on relations

This fixes a rare infinite loop in Mongoid and validation is not needed
anyway.
Save all versions of a formula

Homebrew supports multiple specs (`stable`, `devel` and `head`) for
each formula.

See #22
Improve error messages for import failures
Correctly select formula files in Repository#update_metadata
Rescue from `TypeError`s while importing formulae