We never use system rubygems anymore. Always the local clone.

Instead, use a better rubygems version manager that makes sure that
`ENV["RGV"]` usage is always applied, and its usage is centralized at a
single place.

This simplifies the rake task organization and makes it much simpler to
run locally exactly what's run in CI. For example, since it doesn't rely
on the `RGV` being deleted for subsequent subprocesses, it also running
`bin/rake spec:travis` task locally which previously would lead to

```
rake aborted!

RubyGems version is required on Travis!
```

It also reduces duplication making it so that we only need to change the
`.travis.yml` file when updating tested rubies, and not the `Rakefile`
as well.

7342: Start using automatiek 0.3.0 r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that I added a feature to automatiek to vendor subdependencies, but we were not using it because it had not yet been released.

### What is your fix for the problem, implemented in this PR?

My fix is to start using the feature now that it has been released.

### Why did you choose this fix out of the possible options?

I chose this fix because it allows us to manage the `net-http-persistent` dependency in an easier way.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

If you don't have `ronn` installed, the `man:check` task will fail but
it won't let you know the exact reason.

Since it still has issues.