Fix Python version for Travis CI tests

Change-Id: If558bd48f691902af9541df732cc4f77468b5e19
diff --git a/.travis.yml b/.travis.yml
index 0764388..7f907c9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker