travis: install sexplib (to allow ipaddr.2.5.0 to build)
travis: install sexplib (to allow ipaddr.2.5.0 to build)

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

mirage/ocaml-vchan,mirage/ocaml-vchan,djs55/ocaml-vchan,mirage/ocaml-vchan,djs55/ocaml-vchan,djs55/ocaml-vchan,samoht/ocaml-vchan,samoht/ocaml-vchan,samoht/ocaml-vchan