Remove python 2.6 and cleanup tox.ini

* Remove support for python 2.6
* Remove skipsdist : Needs to be set only if 
  sdist is expensive
* Remove usedevelop : only needed when skipsdist 
  is set to True
* Remove install_command : We can just use the 
  default, we don't need to override
* Remove setenv : We can just use the default as
  we don't need any extra environment variables
* Remove requirements.txt from deps, as this is
  already added automatically


Change-Id: I3564e6d2dfaf64b573382d776e6a79a442750139

tox.ini

@@ -1,21 +1,15 @@
 [tox]
 minversion = 1.6
-skipsdist = True
 envlist = cover,
     docs,
     pep8,
-    py26,
     py27,
     py34,
     pylint,
     update-states
 
 [testenv]
-usedevelop = True
+deps = -r{toxinidir}/test-requirements.txt
-install_command = pip install {opts} {packages}
-setenv = VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/requirements.txt
-    -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:docs]