Test under pypy and pypy3

Author: cfhamlet

.travis.yml

@@ -6,6 +6,10 @@ matrix:
     env: TOXENV=py27
   - python: 3.6
     env: TOXENV=py36
+  - python: pypy
+    env: TOXENV=pypy
+  - python: pypy3
+    env: TOXENV=pypy3
   - env: TOXENV=coverage
 install:
 - pip install -U pip tox codecov