Fix shebangs in apple packages

Our packages were using /usr/bin/env ruby and installing to system ruby
loadpath, which breaks horribly in a multi-ruby (rbenv, etc) environment. This
goes into the workdir and looks for any files dropped in bin, and "seds" the
shebang to /usr/bin/ruby.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>