build.sh: Update docker container to Ubuntu 18.04
build.sh: Update docker container to Ubuntu 18.04

Travis was failing to hit Ubuntu 16.04 repos. The problem went away when
I moved to the newer release. While it probably didn't fix anything,
there's no reason to be on the old release now that the new LTS is out,
so move on up.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>

open-power/pdbg,open-power/pdbg,open-power/pdbg