Make Formula#installed? more idiomatic
Update multi_json to 1.5.0

This contains updates to the OkJson library that allow objects to define
to_json for serialization, and this will be used in the upcoming options
and deps work.
doctor: loosen git origin regexp

If the repository is set up by `brew update`, the remote URL is
https://github.com/mxcl/homebrew.git, but if it is set up by the install
script, the .git suffix is omitted.