Upgrade ruby version for building the dev VM

gds-boxen [1] no longer includes that version. 2.0.0 works for the version
of puppet that we use [2].

[1] https://github.com/boxen/puppet-ruby/tree/master/files/definitions
[2] https://docs.puppetlabs.com/guides/platforms.html#ruby-versions
Merge pull request #528 from alphagov/bump-ruby-version

Upgrade ruby version