Migrate from travis to GitLab CI

.gitlab-ci.yaml

+image: registry.edugit.org/teckids/team-sysadmin/docker-images/python-pimped:master
+
+stages:
+  - test
+
+test:
+  stage: test
+  before_script:
+    - adduser --disabled-password --gecos "Test User" testuser
+    - chown -R testuser .
+  script:
+    - sudo -u testuser coverage run --source=menu_generator manage.py test

.travis.yml

-language: python
-
-python:
-  - "3.8"
-  - "3.7"
-  - "3.6"
-  - "3.5"
-  - "3.4"
-
-env:
-  - DJANGO="django==3.1.3"
-  - DJANGO="django==3.0.11"
-  - DJANGO="django==2.2.17"
-  - DJANGO="django==2.1.15"
-  - DJANGO="django==2.0.13"
-  - DJANGO="django==1.11.29"
-  - DJANGO="django==1.10.8"
-  - DJANGO="django==1.9.13"
-  - DJANGO="django==1.8.19"
-
-matrix:
-  exclude:
-   - python: "3.5"
-     env: DJANGO="django==3.1.3"
-   - python: "3.5"
-     env: DJANGO="django==3.0.11"
-   - python: "3.4"
-     env: DJANGO="django==3.1.3"
-   - python: "3.4"
-     env: DJANGO="django==3.0.11"
-   - python: "3.4"
-     env: DJANGO="django==2.2.17"
-   - python: "3.4"
-     env: DJANGO="django==2.1.15"
-
-install:
-  - pip install $DJANGO
-  - pip install coveralls
-
-script: coverage run --source=menu_generator manage.py test
-
-after_success:
-  coveralls
\ No newline at end of file