update README to latest requirements
Merge branch 'master' of github.com:mirage/mirage
Merge branch 'master' of github.com:mirage/mirage
Only build mirage-types if Io_page is also installed

Fixes failures such as:
https://travis-ci.org/ocaml/opam-repository/jobs/41615656
Merge pull request #324 from avsm/master

Only build mirage-types if Io_page is also installed