Travis: remove Mroonga for clang
Travis: remove Mroonga for clang

Mroonga generated far too many warnings (and hence output) for Travis's
sensibilities on output log file size. So we just remove the storage
engine.

Signed-off-by: Daniel Black <ce6aa2808e3fe4e77f6f1aff6fb988c295097d35@au.ibm.com>

natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2