Add django-polymorphic as test dependency

pytest-factoryboy does not support pytest3.0+

Author: leo-naeka

.travis.yml

@@ -26,8 +26,9 @@ env:
 
   - DJANGO=">=1.10,<1.11" DRF=">=3.4,<3.5"
 before_install:
-  # Force an upgrade of py to avoid VersionConflict
+  # Force an upgrade of py & pytest to avoid VersionConflict
   - pip install --upgrade py
+  - pip install "pytest>=2.8,<3"
   - pip install codecov
 install:
   - pip install Django${DJANGO} djangorestframework${DRF}

setup.py

@@ -105,7 +105,8 @@ def get_package_data(package):
     tests_require=[
         'pytest-factoryboy',
         'pytest-django',
-        'pytest>=2.8',
+        'pytest>=2.8,<3',
+        'django-polymorphic',
     ] + mock,
     zip_safe=False,
 )