ci: use meson 0.47.1

meson 0.53.0 has a compatibility issue [1] with the python 3.5.2 that comes
in Ubuntu 16.04.
On the other hand, the minimal version supported in dpdk is 0.47.1.

Stick to this version to avoid getting hit by regressions in meson latest
shiny release.

1: https://github.com/mesonbuild/meson/issues/6427

Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>