Travis: Bump to Ubuntu Xenial, rationalize build matrix, update CI scripts
Travis: Bump to Ubuntu Xenial, rationalize build matrix, update CI scripts

Bump to Ubuntu Xenial as Solo5 now requires a newer GCC.

Rationalize the build matrix: Test all bindings variants against
the latest compiler (OCaml 4.09.0), only test older compilers against hvt.

Update Travis CI scripts to use the correct repository
(ocaml/ocaml-ci-scripts).

mato/mirage-solo5,mato/mirage-solo5