Add changelog_uri to gemspec

[fynsta]

Supported here: https://guides.rubygems.org/specification-reference/#metadata
Useful for running https://github.com/MaximeD/gem_updater

Thank you for your contribution!

Check the following boxes:

• Wrote good commit messages.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new
  code introduces user-observable changes.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.