v1.4.0

CHANGES:

• Drop the hard dependency on unix that is transitively
  brought in by cstruct/bigarray. This is done by using
  the new bigarray-compat opam package that does the
  right thing on OCaml 4.07+ to not depend on unix.
  On older OCaml compilers the dependency will still be
  there and must be manually dropped with -dontlink, but
  we encourage users to use a newer compiler (#29 @TheLortex)