tools: change distro variable for travis CI
tools: change distro variable for travis CI

ocaml-ci-scripts got changed to construct the image name from DISTRO and
OCAML_VERSION, previously it only used DISTRO.

Signed-off-by: Pau Ruiz Safont <9c4aad32b049b39a74875643d7fe76c19fd4f9a1@citrix.com>

xapi-project/xs-opam,lindig/xs-opam