1.3.1

This is primarily a gem housekeeping release. There should be no user-facing changes.

📚 Documentation

• Fix typo in docs for core:write_release_json task (#166) @mattbrictson

🏠 Housekeeping

• Update rubocop-performance requirement from 1.7.1 to 1.8.0 (#175) @dependabot-preview
• Improve gem packaging practices (#172) @mattbrictson
• Switch to new cimg CircleCI docker images (#171) @mattbrictson
• Bump markdown-include from 0.5.1 to 0.6.0 (#169) @dependabot-preview
• Update rubocop requirement from 0.88.0 to 0.90.0 (#163, #167, #170) @dependabot-preview
• Remove PluginResolver class that wasn't being used (#168) @mattbrictson
• Use CircleCI instead of Travis for testing older Ruby versions (#165) @mattbrictson
• Adopt Contributor Covenant 2.0 (#164) @mattbrictson