travis: switch to bionic build environment
travis: switch to bionic build environment

This is needed in particular to update GPB, as ByteSize() has
been deprecated in favour of ByteSizeLong().

autrimpo/rlite,autrimpo/rlite,autrimpo/rlite,vmaffione/rlite,vmaffione/rlite,vmaffione/rlite,autrimpo/rlite,vmaffione/rlite