add pypy to travis builds

Author: natano

.travis.yml

@@ -5,6 +5,7 @@ python:
     - "2.7"
     - "3.2"
     - "3.3"
+    - "pypy"
 install:
     - "pip install --use-mirrors ."
     - "pip install --use-mirrors -r ./test_requirements.txt"