update travis
update travis

Signed-off-by: Marcello Seri <34fd14c67154e888f6587d5c0e7eadd13aaf7ca7@citrix.com>

djs55/ocaml-xen-lowlevel-libs,djs55/ocaml-xen-lowlevel-libs,djs55/ocaml-xen-lowlevel-libs,djs55/ocaml-xen-lowlevel-libs