Update .gitlab-ci.yml

backend/secfit/.gitlab-ci.yml

-build:
-  stage: build
-  script:
+deploy:
+  stage: deploy
+  image: python:3.8.5-slim
+  before_script:
     - apt-get clean
     - pip install -r backend/secfit/requirements.txt
   cache:
-      paths:
-        - ~/.cache/pip/
+    paths:
+      - ~/.cache/pip/
   services:
     - postgres:latest
-
-deploy:
-  stage: deploy
   script:
     - apt-get update -qy --allow-insecure-repositories
     - apt-get install -y ruby-dev
@@ -19,4 +17,5 @@ deploy:
     - gem install dpl
     - dpl --provider=heroku --app=secfit-group15 --api-key=$HEROKU_API_KEY --skip_cleanup
   only:
-    - master
+    refs:
+      - master