Add git.km3net.de to known_hosts

.gitlab-ci.yml

 before_script:
   # workaround for https://github.com/JuliaDocs/Documenter.jl/issues/686
   - apt-get -qq update; apt-get -y install git
+  - ssh-keyscan git.km3net.de:22 >> /root/.ssh/known_hosts
 
 .script:
   script: