v2.0.0

CHANGES:

• Reinstate OCaml 4.02 to the "recent" list after a request
  from @dra27. This means that all the supported OCaml compilers
  for Dune will be present in a single container.
• Add support for 4.09 now that 4.08 has branched.
• Use +trunk in the suffix for opam dev version packages.
• Add Releases.is_dev to make it easier to spot a dev release.