Update pip to be more resilient to download errors

Update pip and setuptools first to use cached wheels, in order to be more resilient to Travis download errors

tox.ini

@@ -15,6 +15,7 @@ deps =
     nose
     py27: mock
 commands =
+    pip install -U pip setuptools
     pip install -e .
     pip install -e demo
     demo test --cover-package=django_downloadview --cover-package=demoproject {posargs: tests demoproject}