v3.0.0

CHANGES:

• Add support for OCaml 4.11 (#36)
  (Thierry Martinez, review by Gabriel Scherer)
• Add support for OCaml 4.12 (#39)
  (Kate Deplaix, review by Gabriel Scherer)
• Port to ppx_deriving 5.0 and ppxlib (#39)
  (Kate Deplaix, review by Gabriel Scherer)
• Upgrade the tests from ounit to ounit2 (#39)
  (Kate Deplaix, review by Gabriel Scherer)