Update tox.ini to current standards

The sdist step is unecessary (and requires a locally installed pbr).

Change-Id: I1514aab9cdd519442e1e191cf9b8a784b52fc83e

tox.ini

@@ -1,4 +1,6 @@
 [tox]
+minversion = 1.6
+skipsdist = True
 envlist = pep8, py27
 
 [testenv]
@@ -6,6 +8,7 @@ envlist = pep8, py27
 setenv = STATSD_HOST=localhost
          STATSD_PORT=8125
          VIRTUAL_ENV={envdir}
+usedevelop = True
 install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt