Fix Python version for Travis CI tests

Change-Id: I10bc7ff6c0ebc940592cb4f6da79b88ec35e32ba
diff --git a/.travis.yml b/.travis.yml
index 7f03f12..bf99750 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker