Run unit tests on Gitlab CI

.gitlab-ci.yml

+stages:
+  - test
+
+backend-tests:
+  stage: test
+  image: python:3.5-alpine
+
+  services:
+    - postgres:latest
+
+  variables:
+    # For the postgres image
+    POSTGRES_DB: arkindex_dev
+    POSTGRES_USER: devuser
+    POSTGRES_PASSWORD: devdata
+
+    # For the backend
+    DB_HOST: postgres
+    DB_PORT: 5432
+
+  before_script:
+    - apk update
+    - apk add postgresql-dev libjpeg-turbo-dev gcc musl-dev
+    - pip install -r requirements.txt
+
+  script:
+    - cd src && ./manage.py test