travis-ci: run in bash, with the "unofficial strict mode"

travis-ci: run in bash, with the "unofficial strict mode"

set -u forces us to set all variables that we use (for example with the
${foo:=bar} syntax to take an existing value or set a default), or use the
${foo:-bar} syntax to make it explicit that the variable might be unset.

set -o pipefail (which is a bash feature) detects failure in non-last
elements of a pipeline.

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=98889