Fix Python version for Travis CI tests

Change-Id: I071c4ec18c1a03f0a154cb11a2d2436f99f5bd93
diff --git a/.travis.yml b/.travis.yml
index 28d8ddc..db333e3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker