Update CI image

.gitlab-ci.yml

 build-doc:
-    image: gitlab-research.centralesupelec.fr:4567/2017bentrioum/markovprocesses.jl/julia
+    image: julia:latest
     stage: build
     script:
         - "julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd(), subdir=\"core/\")); Pkg.instantiate()'"