Update version in track docs

[kahgoh]

The track's installation guide mentions 4.08, but the test runner and this repository's README uses 5.1. Although the guide only uses 4.08 as an example, we should update the guide to 5.1 to match the test runner and the README to make it clearer (4.14.2 is the current LTS version).

[TheRealOwenRees]

I would like to tackle this over the coming days, updating the track docs to reflect the readme here.

One thing I would like to raise before doing so it a potential bug detailed [here].(#545 (comment))

I had an issue with installing the dependencies while following the readme, with error messages detailing that many dependencies require OCaml < 5.0.0.

I reduced the problem down to two issues:

• using a local switch causes build errors with some packages. You need a global switch.
• the getopts package has dependencies that need OCaml < 5.0.0

Solution
Create a clean global switch for the puropses of the Exercism OCaml environment
opam switch create exercism 5.1.1

Install dependencies manually, omitting getopts and adding other requirements contained within it for the track:
opam install dune ounit qcheck fpath react ppx_deriving ppx_sexp_conv yojson ocp-indent calendar core mustache ezjsonm core_unix

It would be good if someone could test this on their machine with a clean switch and let me know if they have the same issue before proceeding. If not, perhaps this can be added under a troubleshooting header just in case someone else does encounter it.

[TheRealOwenRees]

#547

[kahgoh]

Thanks @TheRealOwenRees! I have a few things going on at the moment, but hope to give this a go later in the week.

[kahgoh]

Fixed in #547