Fix Python version for Travis CI tests

Change-Id: Iafa458d4699b4074122a85f9f3c60f46f07cc9c1
diff --git a/.travis.yml b/.travis.yml
index 7c4a0fa..f4dfa47 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker