Use 'usedevelop' instead of dev install.

tox.ini

@@ -2,10 +2,11 @@
 envlist = flake8, pypy, py26, py27, py32, py33, py34, coverage
 
 [testenv]
+usedevelop=True
 recreate=True
 deps=pytest
-commands=python setup.py dev
-         python setup.py test
+commands=
+  python setup.py test
 
 [testenv:flake8]
 deps=flake8