Add PyPy to travis.
diff --git a/.travis.yml b/.travis.yml
index 05f7d01..f3c3162 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,6 +7,8 @@
   - "3.3"
   - "3.4"
   - "nightly"
+  - pypy
+  - pypy3
 # command to install dependencies
 install:
  - pip install -U pip