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.

So that we don't include a `run` method at the top level, which can
conflict with other `run` methods our specs use.

We're checking the same thing in the previous line

And refactor development setup.

Since it's no longer useful.