[ci] Tox must install dev requirements last

tox.ini

@@ -18,6 +18,7 @@ deps=
 sitepackages = False
 recreate = False
 commands = {toxinidir}/extra/release/removepyc.sh {toxinidir}
+           pip install -q -U -r{toxinidir}/requirements/dev.txt
            nosetests -xsv --with-coverage --cover-inclusive --cover-erase []
 basepython =
     2.7: python2.7