Update tox.ini for new tox 1.6 config

Change-Id: I4363508f562f62b16c856bc072cdb4b37e37b418

tox.ini

@@ -1,7 +1,11 @@
 [tox]
+minversion = 1.6
 envlist = py26,py27,py33,pep8
+skipdist = True
 
 [testenv]
+usedevelop = True
+install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          LANG=en_US.UTF-8
          LANGUAGE=en_US:en
@@ -17,7 +21,7 @@ commands = flake8
 commands = {posargs}
 
 [testenv:cover]
-commands = python setup.py testr --coverage --testr-args='{posargs}'
+commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [tox:jenkins]
 downloadcache = ~/cache/pip