[#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.
Bump rbenv Standards-Version

Fixes lintian warning:

    W: rbenv source: out-of-date-standards-version 3.9.2 (current is 3.9.3)
Remove Vcs-Git and Vcs-Browser from rbenv

Our sources have diverged slightly from upstream referenced by these.
[#42296389] Create shims and versions directories

So that we can set `RBENV_ROOT=/usr/lib/rbenv`.
[#42296389] Upgrade to rbenv 0.4.0

Supports `.ruby-version`, which is what the majority of dev teams are likely
to use, in addition to the older `.rbenv-version` filename.

No longer need to apply hooks patch now that it's included upstream.