4.0.0

CHANGES:

• Port to latest Atd 2.0.0 interfaces (#218 by @mjambon and @avsm)
• Port build system to Dune (@avsm)
• Properly expose the GitHub JavaScript library and fix dependencies (#216 by @samoht)
• Add pull.merge_commit_sha (#217 from @AltGr)
• Convert local opam files to the 2.0 format. (@avsm)