pfff 0.27.1

New version of the parsing libraries (needed by facebook software
soon to be released as open source). This version also works
with ocaml 3.12.1

Test plan:
put this package in my ~/testing repo
opam update testing
opam install pfff
=> it works, it installs pfff-xxx libraries
opam remove pfff
=> it works, it uninstalls pfff-xxx libraries