travis: try running on Bionic
travis: try running on Bionic

Update the Travis environment to use Bionic (18.04) instead of Xenial
(16.04) due to issues installing gcc-multilib.

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core