Fix Python version for Travis CI tests

Change-Id: I1e580e098bc2c20be7f2253fa81fa5370f9ec923
diff --git a/.travis.yml b/.travis.yml
index e58885b..2e5d4b1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker