I think something may have broken or changed with regards to how MariaDB is configured for Xenial.
Passing from yesterday: https://travis-ci.org/coleifer/peewee/jobs/523551111
Erroring from today: https://travis-ci.org/coleifer/peewee/jobs/524146594
Today the following line from my config is failing (only in Xenial):
mysql -e 'grant all on *.* to travis@localhost;' ERROR 1133 (28000) at line 1: Can't find any matching row in the user table
Yesterday this line did not produce any issues in either trusty or xenial.