travis: compile for both Xen 4.2 and 4.4

Signed-off-by: David Scott <dave.scott@citrix.com>
trusty: attempt to install the right version of libxen-dev

Signed-off-by: David Scott <dave.scott@citrix.com>
travis: update list of OCaml and opam versions

Signed-off-by: David Scott <dave.scott@citrix.com>
travis: work around broken libxen-dev package (missing uuid-dev dep)

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #66 from djs55/matrix

travis: compile for both Xen 4.2 and 4.4