Fix Python version for Travis CI tests

Change-Id: I14eec1a61cbe708dc92aad794112922bf8ae1ca6
diff --git a/.travis.yml b/.travis.yml
index cb33408..4b3a46e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker