Fix Python version for Travis CI tests

Change-Id: Ifb619bea83acfa2d01cb000d481e3f72220ad33d
diff --git a/.travis.yml b/.travis.yml
index c7a3dd8..d2e7412 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker