Use python 3.5 with tox.

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py33,py34,report,check
+envlist = py27,py33,py34,py35,report,check
 
 [testenv]
 commands = py.test --cov sigal --cov-report term-missing
@@ -9,7 +9,7 @@ deps =
     pytest-cov
 
 [testenv:report]
-basepython = python3.4
+basepython = python3.5
 commands =
     coverage combine
     coverage report
@@ -17,7 +17,7 @@ usedevelop = true
 deps = coverage
 
 [testenv:check]
-basepython = python3.4
+basepython = python3.5
 deps =
     docutils
     check-manifest