travis: keep meson version below 0.45.0

travis: keep meson version below 0.45.0

Recently Meson upgraded to 0.45.0 and it needs python 3.5+, which is
not available in Trusty.

Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Cc: Dylan Baker <dylan@pnwbakers.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Jon Turney <jon.turney@dronecode.org.uk>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>