Remove python 2.6 and cleanup tox.ini

* Remove support for python 2.6
* 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: I4f650eaa07624b889a52570e628a21e905f2bd46

tox.ini

@@ -1,12 +1,9 @@
 [tox]
 distribute = False
-envlist = py34,py26,py27,pep8
+envlist = py34,py27,pep8
 
 [testenv]
-install_command = pip install -U {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 testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]