Remove merlin.install generation from opam script (fix #245)

merlin.install.sh expects a GNU version of find, which fails on
OSX/BSDs.

This has been fixed, once… The bug was reintroduced. The best is to
remove the script from the installation and precompute merlin.install.
Make merlin.install rule ineffective

To stay compatible with current opam script and still have correct
merlin.install.
Update opam file for OCaml 4.02 RC