Update to ruby-version to 2.4.5
Update to ruby-version to 2.4.5

Since we no longer test gems against Ruby 2.3 [1] it makes sense to bump
this Ruby version.

[1]: https://github.com/alphagov/govuk-jenkinslib/blob/619a7cbb3695f9373d3f8d795bc62d28ca1c5688/vars/govuk.groovy#L655

alphagov/govspeak,alphagov/govspeak