fix ocaml version for recent Coq

1 job for master in 15 minutes and 34 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
failed #46053
fp
build-iris.dev

00:15:34

 
Name Stage Failure
failed
build-iris.dev Build
Makefile.coq:320: recipe for target 'all' failed
make[1]: *** [all] Error 2
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 3m20.379s
user 8m16.920s
sys 0m18.240s
ERROR: Job failed: exit code 1