New

• Support for Ruby GC compaction (#1192)
• Add --with-openssl-dir option for improved linking with OpenSSL (#1303)

Changed

• Lock on the current Fiber rather than current Thread (#1284)
• Improve SSL support on MySQL 5.6.36+ and MariaDB Connector/C 3.0+ (#1304, #1306)
• Additional search paths to find MySQL libraries by Homebrew (#1278)
• README: Improve docs for SSL/TLS (#1142, #1306)
• CI: Upgrade RuboCop and run in its own CI step (#1259, #1268, #1295)
• CI: Update runtime environments (#1290, #1291, #1292, #1298, #1299)

Full Changelog: https://github.com/brianmario/mysql2/compare/0.5.4...0.5.5