Ensure the tests are passed in latest pytest environments

Re #32.

Author: vitalk

.travis.yml

@@ -13,6 +13,8 @@ env:
   - PYTEST=2.5.2
   - PYTEST=2.6.4
   - PYTEST=2.7.0
+  - PYTEST=2.7.1
+  - PYTEST=2.7.2
 
 install:
   - pip install -q pytest==$PYTEST