Update config files to use ocaml 4.13 at least.

.gitlab-ci.yml

@@ -3,13 +3,13 @@ stages:
 
 tests:
   stage: test
-  image: ocaml/opam@sha256:108b775e7bda2c6d3bddae61c9cdb003e40de746baf3a994e8f7b82d8301dd0a
+  image: ocaml/opam@sha256:013a26ccbaa8344b63274e335e2492c432cec1c3526b9ba888ab151abb2b4c25
   cache:
     key: $CI_COMMIT_REF_SLUG
     paths:
       - _opam
   script:
-  - if [ ! -d _opam ]; then echo "no local switch in the CI cache, we setup a new switch"; opam switch create --yes --no-install . ocaml-base-compiler.4.11.1; fi
+  - if [ ! -d _opam ]; then echo "no local switch in the CI cache, we setup a new switch"; opam switch create --yes --no-install . ocaml-base-compiler.4.13.1; fi
   - eval $(opam env)
   - sudo apt-get update
   - sudo apt install -y protobuf-compiler

caisar.opam

@@ -3,7 +3,7 @@ opam-version: "2.0"
 version: "0.1"
 synopsis: "Framework for neural network verification"
 depends: [
-  "ocaml" {>= "4.10"}
+  "ocaml" {>= "4.13.0"}
   "dune-site" {>= "2.9.0"}
   "piqi" {>= "0.7.6"}
   "piqilib" {>= "0.6.14"}

dune-project

@@ -13,7 +13,7 @@
   (name caisar)
   (synopsis "Framework for neural network verification")
   (depends
-   (ocaml (>= 4.10))
+   (ocaml (>= 4.13.0))
    (dune-site (>= 2.9.0))
    (piqi (>= 0.7.6))
    (piqilib (>= 0.6.14))