Travis: several updates

* Avoid compiling OCaml upon opam init. Instead use binary system
  compiler, thereby saving compilation time.

* Make more robust following scripts in
  https://github.com/ocaml/opam-repository.

* Send notifications to biocaml-commits mailing list.

* Set OPAMVERBOSE. Travis times out if there is no output for 10
  minutes, which happens during installation of core_kernel. Setting
  OPAMVERBOSE avoids this at the expense of an absurd amount of
  output.
opam: set version to dev

OPAM’s version ordering guarantees “dev” is higher than any number, so
no need to superficially choose a large number.