Try custom tox install_command

pip_install.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+echo "$@"
+pip wheel "$@"
+pip install --upgrade "$@"

tox.ini

@@ -6,6 +6,7 @@ envlist =
        {py27,py32,py33,py34}-django{17,18beta}
 
 [testenv]
+install_command = ./pip_install.sh {opts} {packages}
 commands = ./runtests.py --fast
 setenv =
        PYTHONDONTWRITEBYTECODE=1