update tox config to not set basepython for zsh

Author: dhellmann

tox.ini

@@ -24,7 +24,6 @@ deps = flake8
 commands = flake8 virtualenvwrapper docs/source/conf.py
 
 [testenv:zsh]
-basepython=python2.7
 setenv =
   SHELL = /bin/zsh
   test_shell_opts = -o shwordsplit