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: I87c1fc87fb2a595b4453be6b33d679f0523cdea8

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 minversion = 1.6
-skipsdist = True
-envlist = docs,py33,py34,py26,py27,pypy,pep8,venv
+envlist = docs,py33,py34,py27,pypy,pep8,venv
 
 [tox:jenkins]
 downloadcache = ~/cache/pip
@@ -11,11 +10,7 @@ basepython = python2.7
 commands = python setup.py build_sphinx
 
 [testenv]
-usedevelop = True
-install_command = pip install {opts} {packages}
-setenv = VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/requirements.txt
-    -r{toxinidir}/test-requirements.txt
+deps = -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]