Fix Python version for Travis CI tests

Change-Id: I4bfc161ad2a0ad6872b945589663f0df10c781ec
diff --git a/.travis.yml b/.travis.yml
index d155429..c685e7c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker