add pypy

Author: wannaphong

.travis.yml

@@ -9,6 +9,8 @@ env:
   - TOXENV=py34
   - TOXENV=py33
   - TOXENV=py27
+  - TOXENV=pypy
+  - TOXENV=pypy3
 # command to install dependencies, e.g. pip install -r requirements.txt --use-mirrors
 install: pip install -U tox