Travis build failed because of very slow MySQL queries execution.
According to MySQL 5.7.23 [release notes](https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-23.html#mysqld-5-7-23-bug)
the `mysql_upgrade` command MUST be run after upgrading to this release.
Otherwise MySQL will flood logs with a warging that results in a performance problems.