Fix Python version for Travis CI tests

Change-Id: I46e9221abfec75743b31919d7258f24c8e957cab
diff --git a/.travis.yml b/.travis.yml
index 17da7a7..723f241 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker