Update .gitlab-ci.yml

Could not import networkx, which somehow strange.
So we do an explicite install

.gitlab-ci.yml

@@ -22,7 +22,7 @@ build:
 test:
   script:
   #- python3 setup.py test
-  - pip3 install --user tox flake8  # you can also use tox
+  - pip3 install --user tox flake8 networkx  # you can also use tox
   #- tox -e py36,flake8
   - python3 -m flake8 navipy/*.py
   - python3 -m unittest discover navipy