bump travis to bionic.
bump travis to bionic.

let us skip out on the OpenSSL build. Can use buildbots to ensure older
distros work.

memcached/memcached,dormando/memcached,memcached/memcached,dormando/memcached,dormando/memcached,memcached/memcached,dormando/memcached,memcached/memcached,memcached/memcached,dormando/memcached