Fix Python version for Travis CI tests

Change-Id: Iee3d328fae0165faea0e00c0116206d046ec74f4
diff --git a/.travis.yml b/.travis.yml
index ea20576..3f93beb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker