.travis.yml: Purge mysql-common
.travis.yml: Purge mysql-common

The config file /etc/mysql/my.cnf confuses the brewed mysql.

maxim-belkin/homebrew-core,rwhogg/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core