Use opam2 in travis scripts

[samoht]

No description provided.

[samoht]

/cc @AltGr @rjbou

[samoht]

The error on 4.00.1 is:

[ocaml-base-compiler.4.00.1] downloaded from http://caml.inria.fr/pub/distrib/ocaml-4.00/ocaml-4.00.1.tar.gz
[ERROR] The sources of the following couldn't be obtained, aborting:
          - ocaml-base-compiler.4.00.1: Bad checksum

Which is due to the fact the the associated patch has a bad checkseum. Should I push a fix to that hash as well?

[camelus]

🌤️ opam-lint warnings 4c559c3

• ocaml-base-compiler.4.00.1 has some warnings:

  • warning 25: Missing field 'authors'
  • warning 35: Missing field 'homepage'
  • warning 36: Missing field 'bug-reports'
  • warning 37: Missing field 'dev-repo'

---

☀️ Installability check (9500 → 9500)

[samoht]

TravisCI is green and the script has been tested successfully with #12444 so merging.