Fix Python version for Travis CI tests

Change-Id: I40a4f2135dd3974b1e50620b6e950daf59f58f30
diff --git a/.travis.yml b/.travis.yml
index 14e83f0..c4a78d7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker