alt-ergo-free 2.2.0 is not compatible with ocaml5
```
File "/home/opam/.opam/5.0/.opam-switch/build/alt-ergo-free.2.2.0/lib/util/myUnix.ml", line 19, characters 7-22:
19 | if Pervasives.(<>) timelimit 0. then
^^^^^^^^^^^^^^^
Error: Unbound module Pervasives
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>