Add MySQL 5.7 test to CI.

preview: https://travis-ci.org/zizkovrb/mysql2/builds/38104527
based on: http://unix.stackexchange.com/questions/158052/how-to-configure-the-mysql-apt-repo-on-ubuntu-on-a-non-interactive-shell

related to https://github.com/brianmario/mysql2/issues/548#issuecomment-59258039