Rather than having to manually keep track of what version each thing in
here is and copy files around by hand on update let's use Bundler's
standalone mode and careful use of `.gitignore` to help us do it.

This means a `bundle update --standalone` will allow us to update all
gems in vendor.

We could consider vendoring other gems this way in future but I'd
suggest only doing this for gems with no dependencies or at least gems
with no native extensions. The only gem this applies to that we
currently use is `ruby-prof` and I'm not convinced it's widely used
enough to warrant vendoring for everyone. Perhaps that's another
criteria: it should be functionality that's used by non-developer
commands and/or normal Homebrew usage.