Fix Python version for Travis CI tests

Change-Id: I2d33f887125167ce625d2ec9ee68532a3940f632
diff --git a/.travis.yml b/.travis.yml
index 45062b7..2a2c40c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+language: python
+python:
+- "2.7.13"
 sudo: required
 services:
   - docker