Update makefile to use flake8 instead of pyflakes

Makefile

 	# Run continous tests and generate lint reports
 	python setup.py develop
 	pip install -r requirements.txt
-	#./runtests.py --with-coverage
-	pyflakes oscar | perl -ple "s/: /: [E] /" | grep -v migrations > violations.txt
-	pep8 --exclude="migrations" oscar | perl -ple 's/: [WE](\d+)/: [W$1]/' > violations.txt
+	./runtests.py --with-coverage
+	flake8 oscar | perl -ple "s/: /: [E] /" | grep -v migrations > violations.txt
 
 test:
 	./runtests.py

requirements.txt

 Whoosh==2.4.1
 WebTest==1.3.4
 django-webtest==1.5.3
+pyflakes==0.5.0