Make ruby 2.3.1 available
Make ruby 2.3.1 available

It's a bugfix release so we should use it over 2.3.0 if possible, but also
the govspeak gem specifies 2.3.1 explicitly so having it available in the
dev vm is useful.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet