tox: Don't need to install to run flake8

This should fix the broken Travis build

tox.ini

@@ -15,4 +15,5 @@ commands = nosetests -v --with-xunit --xunit-file=xunit-{envname}.xml --with-cov
 deps =
     flake8
     flake8-import-order
-commands = flake8
+skip_install = true
+commands = flake8