Fix travis virtualenv bug

Author: cbetta

.travis.yml

@@ -10,7 +10,9 @@ branches:
   only:
     - master
     - /^v[0-9]/
-before_install: pip install tox-travis
+before_install:
+  - pip install virtualenv==15.2.0 
+  - pip install tox-travis
 script: tox
 before_deploy:
   - pip install -e .