Revert "Removed py3+ tests in tox"

This reverts commit 2c720b05ae.

tox.ini

@@ -6,7 +6,7 @@
 # acme and letsencrypt are not yet on pypi, so when Tox invokes
 # "install *.zip", it will not find deps
 skipsdist = true
-envlist = py26,py27,cover,lint
+envlist = py26,py27,py33,py34,cover,lint
 
 [testenv]
 commands =
@@ -23,6 +23,16 @@ setenv =
     PYTHONHASHSEED = 0
 # https://testrun.org/tox/latest/example/basic.html#special-handling-of-pythonhas
 
+[testenv:py33]
+commands =
+    pip install -e acme[testing]
+    nosetests acme
+
+[testenv:py34]
+commands =
+    pip install -e acme[testing]
+    nosetests acme
+
 [testenv:cover]
 basepython = python2.7
 commands =