Fix Python version for Travis CI tests

Change-Id: Ifa1f20c4bbb2d4c64530ff5668bd2f54db6e8d56
diff --git a/.travis.yml b/.travis.yml
index c4efd76..d836d6a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker