Update .gitlab-ci.yml

Configured tes_coverage and test_pylint to change to backend dir

.gitlab-ci.yml

-
 stages:
         - build
         - test
@@ -25,13 +24,15 @@ test_coverage:
         services:
                 - postgres:9.5
         script:
-                - coverage run backend/manage.py test --noinput
+                - cd backend/
+                - coverage run manage.py test --noinput
                 - coverage report --skip-covered
 
 test_pylint:
         <<: *test_definition
         script:
-                - pylint backend/core || exit 0
+                - cd backend/
+                - pylint core || exit 0
 
 
 .staging_template: &staging_definition