added pypy and pypy3 testing to Travis CI

Author: chrissimpkins

.travis.yml

@@ -14,6 +14,10 @@ matrix:
       env: TOX_ENV=py35
     - python: 3.6
       env: TOX_ENV=py36
+    - python: pypy
+      env: TOX_ENV=pypy
+    - python: pypy3
+      env: TOX_ENV=pypy3
 
 script: tox -e $TOX_ENV