Update tox requirements

Update to needing 1.6, which gives us the ability to alter how the
software is installed into the virtualenv. It also brings in pip 1.4,
which lets us avoid getting pre-releases of things we weren't expecting.

Change-Id: I3189f06610d776a032b5f8bf0910f59e4ed45719

tox.ini

@@ -1,7 +1,11 @@
 [tox]
+minversion = 1.6
+skipsdist = True
 envlist = py26,py27,py26sa07,py27sa07,py33,pep8
 
 [testenv]
+usedevelop = True
+install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt