[opam] Update to OPAM 2.0 + allow testing of experimental compiler versions

[ejgallego]

• we add the ocaml-pr-repository switch as to allow testing of experimental copmiler versions.
• we disable the install of camlp5 to make the above work.

We should add a coq_dependencies field so we could go back to testing older Coq versions, even tho #58 would make this unnecessary.

[ejgallego]

I am very confused, despite the call to OPAM using -j2, Coq is not built in parallel:

/usr/bin/make "-j" "1" (CWD=/home/travis/build/coq/coq-bench/_workspace/travis/opam.NEW/4.07.1/.opam-switch/build/coq.dev)

https://travis-ci.com/coq/coq-bench/jobs/172444011#L1052

Any idea of what is going on?

[gares]

You should try opam config set-global jobs 2 since the value it infers is 1 (dunno why). The inferred value is the max it uses, hence passing -j2 is useless (as far as I understand the doc). You could also export OPAMJOBS if you prefer

[ejgallego]

You should try opam config set-global jobs 2 since the value it infers is 1 (dunno why). The inferred value is the max it uses, hence passing -j2 is useless (as far as I understand the doc). You could also export OPAMJOBS if you prefer

Indeed that seems the best solution; thanks for the tip about the value being the max; I was not aware of that and it explains a lot of things.

[ejgallego]

Ok, pushed again with @gares workaround, let's see if things work this time.

[gares]

it works!

[ejgallego]

Finally ready, merging then. Thanks for the help!