Add a reinstall_dependencies script, update README

I got annoyed at running the reinstall dependencies commands, especially
when I discovered that puppet manifests are cached to .tmp/librarian.

Also for the Ruby uninitiated it's more efficient for the README to
gently signpost towards rbenv and ruby-build than let them struggle with
their out-of-date system installed ruby :)
Merge pull request #186 from alphagov/dependencies-tool

Add a reinstall_dependencies script, update README