Fix: MySQL 5.6.16 が入っているのに　5.6.15 を入れようとしてエラーになる

### エラーメッセージ
```
Error: Could not update: Execution of 'brew boxen-upgrade mysql' returned 1: Error: mysql-5.6.16 already installed
Error: /Stage[main]/Mysql::Install/Package[mysql]/ensure: change from 5.6.16 to 5.6.15 failed: Could not update: Execution of 'brew boxen-upgrade mysql' returned 1: Error: mysql-5.6.16 already installed
```