Add deriving opam packages back

[georgyo]

These packages got dropped in #50, but are needed by tests see exercism/ocaml#496

[ErikSchierboom]

@georgyo Sorry to hear about that. I removed them because they couldn't be installed (see the CI output):

 > [runner 3/6] RUN opam install base ounit2 ppx_deriving ppx_sexp_conv qcheck react calendar:
6.842   - ppx_sexp_conv -> ppx_core -> base < v0.11 -> ocaml < 4.07.0
6.842       base of this switch (use `--unlock-base' to force)
6.842   - ppx_sexp_conv -> ppx_tools >= 0.99.3 -> ocaml < 5.1.0
6.842       base of this switch (use `--unlock-base' to force)
6.842   - ppx_sexp_conv -> ppxlib (< 0.9.0 | >= 0.23.0) -> ocaml < 5.2.0
6.842       base of this switch (use `--unlock-base' to force)
6.842   - ppx_sexp_conv -> sexplib0 < v0.15 -> ocaml < 5.0
6.842       base of this switch (use `--unlock-base' to force)

Any idea how to fix this?

[georgyo]

I see the problem, Ocaml 5.2 isn't released yet and won't be for several months to a year.

5.1 is the latest stable release: https://ocaml.org/releases

Add a commit to set the ocaml version to 5.1 and this time verified that it builds and tests locally.

[ErikSchierboom]

Thanks!