Homebrew: use env=std by default.
Homebrew: use env=std by default.

This causes Homebrew to set the appropriate LDFLAGS for Readline, which
is a keg only formula.

Without this, gitsh can't build against system Ruby (although it still
worked on systems where Homebrew's ruby formula was installed).

I couldn't find good documentation for the difference between `env=std`
and `env=superenv`, but this is where the necessary LDFLAGS value is set:

  https://github.com/Homebrew/brew/blob/89fd34b/Library/Homebrew/build.rb#L100

thoughtbot/gitsh,thoughtbot/gitsh,thoughtbot/gitsh