Fix Python version for Travis CI tests

Change-Id: Ie240d54a72f5cee171ea3caab44f2fe4ae4f79b3
diff --git a/.travis.yml b/.travis.yml
index a13b402..d139081 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker