Fix Python version for Travis CI tests

Change-Id: I58afe71799267fe4ff6067fcb0b312a39f4f2fce
diff --git a/.travis.yml b/.travis.yml
index e2e70d4..7b079bf 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker