fix travis.yml

.travis.yml

       before_install:
         - docker pull mailhog/mailhog
         - docker run -d -p 1025:1025 -p 8025:8025 mailhog/mailhog
+        - cd backend
       install:
         - pip install --upgrade pip setuptools
         - pip install -e ".[testing]"
         - pip install pytest-cov
         - pip install python-coveralls
 
+      before_script:
+       - cd backend
       script:
-       - backend/py.test --cov tracim_backend
+       - py.test --cov tracim_backend
 
       after_success:
         - coveralls