Tox: Stop using -v for py.test

tox.ini

@@ -15,7 +15,7 @@ deps=
 sitepackages = False
 recreate = False
 commands = pip install -U -r{toxinidir}/requirements/dev.txt
-           py.test -xv --cov=celery --cov-report=html
+           py.test -x --cov=celery --cov-report=html
 
 basepython =
     2.7,flake8,flakeplus,apicheck,linkcheck,configcheck: python2.7