Release v2.1

CHANGES:

• Upgrade to ppxlib 0.18.0 (#8, @NathanReb)

ppx_getenv 2.0

• Switch from topkg/ocamlbuild to dune (#6 @kit-ty-kate)
• Switch from ppx_tools to ppxlib (#6 #7 @kit-ty-kate)

These changes bring OCaml 4.11 support as well as the ability to use ppx_getenv directly in dune.

v1.0

OASIS → topkg (fixes #1).