Upgrade the Nokogiri gem

[wendelfabianchinsamy]

Purpose

closes: https://github.com/datacite/bolognese/issues/191

Approach

Open Questions and Pre-Merge TODOs

Learning

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)

• [ ] New feature (non-breaking change which adds functionality)

• [x] Breaking change (fix or feature that would cause existing functionality to change)

Reviewer, please remember our guidelines:

• Be humble in the language and feedback you give, ask don't tell.
• Consider using positive language as opposed to neutral when offering feedback. This is to avoid the negative bias that can occur with neutral language appearing negative.
• Offer suggestions on how to improve code e.g. simplification or expanding clarity.
• Ensure you give reasons for the changes you are proposing.

[wendelfabianchinsamy]

A minor comment, in the gemspec, do we want to update 1 s.required_ruby_version = ['>=2.3']`??

This shows up in rubygems.org

Great point @svogt0511. I will change this to 3.0.