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.

And refactor development setup.

Currently the first time you run `bin/rake spec:deps` on a fresh
installation, you get the following output:

```
Could not find 'automatiek' (~> 0.1.0) among 17 total gem(s)
Checked in 'GEM_PATH=/usr/local/rvm/gems/ruby-2.3.7:/usr/local/rvm/gems/ruby-2.3.7@global', execute `gem env` for more information (Gem::MissingSpecError)
Could not find 'mustache' (= 0.99.6) among 17 total gem(s)
Checked in 'GEM_PATH=/usr/local/rvm/gems/ruby-2.3.7:/usr/local/rvm/gems/ruby-2.3.7@global', execute `gem env` for more information (Gem::MissingSpecError)
Could not find 'rake' (~> 12.0) - did find: [rake-10.4.2]
Checked in 'GEM_PATH=/usr/local/rvm/gems/ruby-2.3.7:/usr/local/rvm/gems/ruby-2.3.7@global', execute `gem env` for more information (Gem::MissingSpecVersionError)
Could not find 'rdiscount' (~> 2.2) among 17 total gem(s)
Checked in 'GEM_PATH=/usr/local/rvm/gems/ruby-2.3.7:/usr/local/rvm/gems/ruby-2.3.7@global', execute `gem env` for more information (Gem::MissingSpecError)
Could not find 'ronn' (~> 0.7.3) among 17 total gem(s)
Checked in 'GEM_PATH=/usr/local/rvm/gems/ruby-2.3.7:/usr/local/rvm/gems/ruby-2.3.7@global', execute `gem env` for more information (Gem::MissingSpecError)
Could not find 'rspec' (~> 3.6) among 17 total gem(s)
Checked in 'GEM_PATH=/usr/local/rvm/gems/ruby-2.3.7:/usr/local/rvm/gems/ruby-2.3.7@global', execute `gem env` for more information (Gem::MissingSpecError)
Could not find 'rubocop' (= 0.50.0) among 17 total gem(s)
Checked in 'GEM_PATH=/usr/local/rvm/gems/ruby-2.3.7:/usr/local/rvm/gems/ruby-2.3.7@global', execute `gem env` for more information (Gem::MissingSpecError)
```

Nothing has gone wrong but the user might think the opposite. So, avoid
printing these errors.