Add --HEAD support to Homebrew formula

This change adds support for --HEAD builds to the Homebrew formula. It
defines a conditional dependency on Maven and builds the -cli project
during install.  The location to install from is abstracted out so
that code is identical regardless of whether the build is --HEAD or
mainline.
Ruby indentation conventions

By convention, Ruby code is indented using two spaces (  ) per-level.
The code used tabs (\t) previously and therefore wasn't idiomatic.
This change updates the indenting to use two spaces.