build: add Python 3.5 to tox environments

* behave stopped working with Python 2.6 with v1.2.5, lacking
  a dependency for `importlib`. Fix this by adding it to the tox
  dependencies list.

Author: Steve Canny

tox.ini

@@ -11,7 +11,7 @@ python_classes = Test Describe
 python_functions = it_ they_
 
 [tox]
-envlist = py26, py27, py33, py34
+envlist = py26, py27, py33, py34, py35
 
 [testenv]
 deps =
@@ -25,6 +25,15 @@ commands =
     py.test -qx
     behave --format progress --stop --tags=-wip
 
+[testenv:py26]
+deps =
+    importlib>=1.0.3
+    behave
+    lxml
+    mock
+    pyparsing
+    pytest
+
 [testenv:py33]
 deps =
     behave