Fix Python version for Travis CI tests

Change-Id: I9ce5616c5f2c62aae7f048575c9986638fdede9a
diff --git a/.travis.yml b/.travis.yml
index c8024aa..11fc963 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker