[#42296389] rbenv sources from Ubuntu Raring

Stock contents of `rbenv_0.3.0-1.debian.tar.gz` from:

- https://launchpad.net/ubuntu/+source/rbenv/0.3.0-1
[#42296389] srcurl and make targets for rbenv

Uses GitHub as mirror instead of Launchpad. I assume this is safer.
[#42296389] Convert rbenv-alternatives to a patch

This makes it easier to track in our repo. Compared to be an additional
source tarball.
[#42296389] Remove rbenv-alternatives

We don't actually have a need for this, as we only ever want to use a single
system Ruby binary. The patch also isn't in this upstream version of rbenv
so removing it simplifies our install.

NB: There was a permissions issue which I fixed with `override_dh_fixperms`
to set execute bits on the rbenv-alternatives binary. If we reinstate this
with a patch again then we'll need to do that.