Fix Python version for Travis CI tests

Change-Id: I24c2fda012fb3159cc858da6a9cf388975cab6b6
diff --git a/.travis.yml b/.travis.yml
index 9f61103..517a8f9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker