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.