Get the Gemfile from GitHub if it's not already there.

When the jekyll role is applied to jenkins, there's no source tree to find a `Gemfile` or `Gemfile.lock` from, as described in #147. This should:

 * Use the `Gemfile` and `Gemfile.lock` from the source tree, if a source tree is present, which will be the case in Vagrant.
 * Download the latest `Gemfile` and `Gemfile.lock` from GitHub's raw content API to `/tmp` if no source tree is specified, which will be the case on the production Jenkins box.

This should let us use the same role for both cases. It is a *bit* of a hack, but it should satisfy all of the cases we need to cover.