Fix Python version for Travis CI tests

Change-Id: I2282b20eded1b3214468d8b2985b18dc3e9a9d2a
diff --git a/.travis.yml b/.travis.yml
index f58cc31..0846964 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker