Add ability to use puppet from git via bundler

Add ability to use puppet from git via bundler

Without this patch the Gemfile can only satisfy dependencies using
officially release gem versions.  This is a problem because we want to
test stdlib against the latest HEAD of the puppet git repository.

This patch addresses the problem by copying over the location_for method
from the Puppet Gemfile, which will parse out git://, file:// or Gem
version specifications.

This, in turn, allows jobs to be configured to run with different
dependency integrations.