Use setup.py develop for tox install.

tox 1.6 was released, so, we can use new develop mode that'll speed up
project installation.

Change-Id: I42a34bf65a78bb200804f11775de279624771ecd

tox.ini

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