Fix MySQL 8.0 CI installing MySQL 5.5

MySQL 8.0 Community APT packages are only available from 16.04, so we
need to use "dist: xenial" on Travis CI.

Author: felixbuenemann

.travis.yml

@@ -65,6 +65,7 @@ matrix:
           - mysql2gem.example.com
     - rvm: 2.4
       env: DB=mysql80
+      dist: xenial
       addons:
         hosts:
           - mysql2gem.example.com