Make deprecation warnings error on Travis

Missed in django-import-export#1197, which changed only the tox configuration and not for Travis.

Author: adamchainz

.travis.yml

@@ -59,7 +59,7 @@ install:
   - pip install -r requirements/test.txt
   - pip install coveralls
 script:
-   - PYTHONPATH=".:tests:$PYTHONPATH" python -Wall -m coverage run --omit='setup.py' --source=. tests/manage.py test core --settings=
+   - PYTHONPATH=".:tests:$PYTHONPATH" python -Wall -W error::DeprecationWarning -W error::PendingDeprecationWarning -m coverage run --omit='setup.py' --source=. tests/manage.py test core --settings=
    - if python -c 'import sys; sys.exit(1 - (sys.version_info >= (3, 6)))'; then isort --check-only import_export tests; fi
 after_success:
   - coveralls