Update installation instructions

[carlosdagos]

Followed instructions and found a bit of an issue, so I updated them to work as per noted in https://github.com/ocaml/opam/issues/3815

The rest seemed to work fine save for some libraries I was missing (like gmp, libcurl, ncurses, etc). I noticed that the travis scripts actually uses 4.07.1, would that be relevant for the docs as well or is that still work in progress? Thank you!