travis: unset OPAM_VERBOSE to generate less logging

Travis seems to have a limit on the amount of logging it's prepared
to capture.

Signed-off-by: David Scott <dave.scott@citrix.com>
travis: add dependency on mirage-xen-minios

Signed-off-by: David Scott <dave.scott@citrix.com>
travis: Only OCaml 4.01.0 is supported

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #98 from djs55/fix-travis

travis: unset OPAM_VERBOSE to generate less logging