Fix Python version for Travis CI tests

Change-Id: I5d3e7266ce117d632909f24180705914987953c6
diff --git a/.travis.yml b/.travis.yml
index c506297..16411a7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker