Fix Python version for Travis CI tests

Change-Id: I06ff9ea44ddee2b479b9c58c0564dfb67152b6c6
diff --git a/.travis.yml b/.travis.yml
index 20efff9..d3caa06 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker