Fix Python version for Travis CI tests

Change-Id: Idd52f0285ca3c858d21850f8d324cdfe64e149d6
diff --git a/.travis.yml b/.travis.yml
index 52b0e80..f7ac517 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker