Update ocaml-sexplib to 111.25, otherwise jessie's current version gazumps us

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Jessie now has libxen-4.4

Fixes #557

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add a 'Status by distro' section

Fixes #557

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #563 from djs55/attempt-to-fix-jessie

Fix build on jessie