Updated OCurl

The binding to curl is no longer compiling on modern version of OCaml.
Fork have been done by others (like @williamleferrand). I have just forked his fork (yeah…) to modernize a bit the _oasis file and release this version. 

I’ll try to get in touch with the original author in order to see if it is possible for him to release a fixed version. In the meantime, I feel like opam users are better of with a version that works than something not working.