Updated package to latest version, 2.0.0beta4. Changes include:

* Building with OCaml < 4.02.3 is no longer supported
* Support compilation with a built-in mccs solver (removing run-time dependency
  to aspcud). Integrated half-working "heuristics" dropped.
* Remove jsonm and transitive uutf dependency.
* Switch build systems to `jbuilder`, including `lib-ext` support
* Allow repeated and or'd arguments to the `list` command
* Many code and build system portability fixes (Windows, BSD)
* Add `switch export --full` to include package definitions
* Add `announce:` and `stamp:` fields to repositories (`repo` files)
* Add a global cache of git objects (greatly speeding up multiple cloning)
* Allow `opam pin URL` without a package name
* Many error handling, messages and corner case fixes
* Updated the versions of dependencies
* Removed the unused `features:` field, and proposal for a new syntax
* More informative exit codes, and documentation thereof

(jaapb)