Merge branch 'ci' into 'master'

CI Script

See merge request funkwhale/mopidy!1

.gitlab-ci.yml

+stages:
+  - test
+
+test:
+  stage: test
+  image: python:2
+  before_script:
+    - pip install .[test]
+  script:
+    - pytest
+  tags:
+    - docker