Use develop install and fix docutils version

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

docutils version should be fixed to prevent doc build failures.

Change-Id: Icb713f54fc7646d7325d66dc26a5f91250451a4e

test-requirements.txt

@@ -2,6 +2,7 @@ hacking>=0.6,<0.8
 
 coverage
 discover
+docutils==0.9.1
 fixtures>=0.3.12
 jsonschema
 mock

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}
 setenv = VIRTUAL_ENV={envdir}
          LANG=en_US.UTF-8
          LANGUAGE=en_US:en