Fix gitlab's submodule strategy

gitlab-ci/latex.yml

 compile_latex:
   image: aergus/latex # the docker image by aergus https://hub.docker.com/r/aergus/latex/
   stage: build
-  variables:
-    GIT_SUBMODULE_STRATEGY: recursive
   before_script:
+    - git submodule sync
+    - git submodule update --init --remote
     # if you need to upload PDFs somewhere else, add a private  SSH key to the
     # gitlab project config and then do the following:
     #- 'which ssh-agent || ( apt-get update -y && apt-get install openssh-client -y )'