Add pytest-warnings to test reqs

.travis.yml

@@ -10,6 +10,6 @@ install:
   - pip install -r test-requirements.txt
   - python setup.py develop
   - pip freeze
-script: py.test --cov=./
+script: py.test -r w --cov=./
 after_success:
   - codecov

test-requirements.txt

@@ -3,3 +3,4 @@ factory_boy
 codecov
 coverage
 pytest-cov
+pytest-warnings