Use new io-page ocamlfind package names

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
travis: io-page-{unix,xen} are superceded by io-page

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #18 from djs55/new-io-page

Use new io-page ocamlfind package names