Fix Python version for Travis CI tests

Change-Id: Idcdeee2165deb988c68b2522f83aa73f6a7a3fb2
diff --git a/.travis.yml b/.travis.yml
index 0f978f7..28df064 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker