Improve homebrew Ruby dependencies.
Improve homebrew Ruby dependencies.

We can depend on Homebrew Ruby if it's already installed, even if system
Ruby is new enough. Homebrew Ruby is generally better for gitsh, because it
has readline instead of libedit.

jakirkham/gitsh,jakirkham/gitsh,jakirkham/gitsh,andybalaam/gitsh,andybalaam/gitsh,andybalaam/gitsh,thoughtbot/gitsh,adrianheine/gitsh,adrianheine/gitsh,thoughtbot/gitsh,thoughtbot/gitsh