Fix travis 7.2+MySQL 5.7 tests

[wolfy-j]

Sounds like the distro was updated and this dependency is not available anymore.

[wolfy-j]

#     # MySQL 5.7
#     - stage: Test
#       php: 7.2
#       addons:
#         apt:
#           sources:
#             - mysql-5.7-trusty
#           packages:
#             - mysql-server
#             - mysql-client
#       env:
#         - "DB=mysql MYSQL=5.7"
#       before_install:
#         - sudo mysql -e "use mysql; update user set authentication_string=PASSWORD('root') where User='root'; update user set plugin='mysql_native_password';FLUSH PRIVILEGES;"
#         - sudo mysql_upgrade -u root -proot
#         - sudo service mysql restart
#         - mysql -u root -proot -e 'CREATE DATABASE spiral;'
#       script:
#         - vendor/bin/phpunit tests/ORM/Driver/MySQL