Fix Python version for Travis CI tests

Change-Id: I446ea973f725a1d538687f0e8e28de471b796dd0
diff --git a/.travis.yml b/.travis.yml
index f44c2ff..a4b9d65 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker