Fix Python version for Travis CI tests

Change-Id: Id34e9b9978f77f61cfe9eb2562212bd3e1bdd165
diff --git a/.travis.yml b/.travis.yml
index baee2fc..2982ea2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker