Fix Python version for Travis CI tests

Change-Id: I8c694b22a683effbc945d8e0581d717e99e76a4b
diff --git a/.travis.yml b/.travis.yml
index fe7fbd1..0cbcc03 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker