Fix Python version for Travis CI tests

Change-Id: I4ab6f58d06aebd7338c784999c74ab95bd0a6cc2
diff --git a/.travis.yml b/.travis.yml
index e7b61ad..6ee89cc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker