Fix Python version for Travis CI tests

Change-Id: I422594de6d52e706658caa3c306df2fad3c5dbe6
diff --git a/.travis.yml b/.travis.yml
index fae782f..b15f211 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker