Add Object Caching tests to Travis build

[moorscode]

While looking at the Travis configuration for the WordPress-develop repository, I noticed the build which uses Memcached as object cache implementation.

We need to implement this build as well, to make sure nothing breaks.

Related:
https://github.com/WordPress/wordpress-develop/blob/master/.travis.yml#L24 - Add memcache service
https://github.com/WordPress/wordpress-develop/blob/master/.travis.yml#L45 - Trigger needed scripts/files