Fix Python version for Travis CI tests

Change-Id: I09d322ab521c6d84d0966bc7e503ff4531c21e18
diff --git a/.travis.yml b/.travis.yml
index 36eea65..0bd4d62 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker