Fix mysql-5.7 test case's error in Travis.

See https://travis-ci.org/brianmario/mysql2/jobs/615263122#L798

```
+ add-apt-repository 'http://repo.mysql.com/apt/ubuntu mysql-5.7'
+ apt-get install -qq mysql-server libmysqlclient-dev
```

After adding trusty mysql-5.7 repository, mysql-5.5 is installed
instead of mysql-5.7.
because the Packages file is empty (zero byte).
http://repo.mysql.com/apt/ubuntu/dists/trusty/mysql-5.7/binary-amd64/

xenial mysql-5.7 repository is still available.
The Packages file is not empty.
http://repo.mysql.com/apt/ubuntu/dists/xenial/mysql-5.7/binary-amd64/

.travis.yml

@@ -60,6 +60,7 @@ matrix:
           - mysql2gem.example.com
     - rvm: 2.4
       env: DB=mysql57
+      dist: xenial
       addons:
         hosts:
           - mysql2gem.example.com