Add --with-xunit to nose test runner for CI

Makefile

@@ -17,7 +17,7 @@ ci:
 	# Run continous tests and generate lint reports
 	python setup.py develop
 	pip install -r requirements.txt
-	./runtests.py --with-coverage
+	./runtests.py --with-coverage --with-xunit
 	flake8 oscar | perl -ple "s/: /: [E] /" | grep -v migrations > violations.txt
 
 test: