Update following io-page ocamlfind package rename

Instead of
  io-page-unix
  io-page-xen

We now have
  io-page
  io-page.unix (containing only the extra C stub)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update CHANGES

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

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #82 from djs55/new-io-page

Update following io-page ocamlfind package rename