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
Move latest formula change down
Make use of Formula#update_metadata in Repository#recover_deleted_formulae
Ignore exceptions in eval'd old Homebrew code
Fix Repository spec to support multiple versions
Test all of Formula’s methods
Add the missing spec for Formula#generate_history!
Simplify how commit information is processed
Simplify how formulae information is processed
Reuse Repository#reset_head
Simplify recovery of deleted formulae
Fix broken Repository#refresh
Hide removed formulae revdeps
Mark removed formulae in lists

This is currently only used in formula dependencies. If you see this,
there's probably a problem in the repository, i.e. the formula can't be
installed without its dependencies.