databases/ruby-mysql2: update to 0.5.1

0.5.1 (2018/04/11)

New Features

* None

Bug Fixes

* Fix with --with-mysql-dir (#952)
* Prevent command out of sync errors with Prepared Statements (#956, #957, #958)

Changes

* Specs: Use the prepared statement performance schema if available (#960)
* README mysql2 0.5.x works with Rails 5.0.7, 5.1.6, and higher
* README be sure to read about the known limitations of prepared statements

(taca)