Use complex factor conditions in tox.ini

`!py26` would be nice but that wasn't introduced until 3.0.0rc1 and
it seems polite to not depend on pre-release versions of tox.

Author: altendky

tox.ini

@@ -7,9 +7,11 @@ envlist=
 
 [testenv]
 deps=
-    pytest
     greenlet
-    twisted
+    py26: pytest<3.3
+    py{27,34,35,36}: pytest
+    py26: twisted<15.5
+    py{27,34,35,36}: twisted
     qt5reactor: pytest-qt
     qt5reactor: qt5reactor
     qt5reactor: pytest-xvfb
@@ -20,14 +22,6 @@ setenv=
 commands=py.test --reactor={env:reactor_option} []
 sitepackages=False
 
-[testenv:py26-defaultreactor]
-deps=
-    pytest<3.3
-    greenlet
-    twisted<15.5
-commands=py.test []
-sitepackages=False
-
 [testenv:linting]
 deps=flake8
 commands=flake8