Update tox and travis configs

I'm removing python2.6 from travis for the time being. It
requires unittest2 and it's not clear how to install that
for just 2.6 in travis.

.travis.yml

@@ -1,12 +1,12 @@
 language: python
 
 python:
-  - 2.6
   - 2.7
-  - 3.2
   - 3.3
   - 3.4
+  - 3.5
   - pypy
+  - pypy3
 
 services:
   - mongodb

tox.ini

@@ -4,7 +4,8 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py26, py27, py32, py33, py34, pypy
+envlist = py26, py27, py33, py34, py35, pypy, pypy3
+skip_missing_interpreters = True
 
 [testenv]
 commands =