Fix Python version for Travis CI tests

Change-Id: I6b37adb81766abd2f27da7ada7ba14681b47e07f
diff --git a/.travis.yml b/.travis.yml
index b9ee103..533ccac 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker