Fix travis on python2.6

.travis.yml

@@ -1,9 +1,10 @@
 language: python
+# Workarround to make 2.7 use system site packages, and 2.6 not use system
+# site packages.
+# https://github.com/travis-ci/travis-ci/issues/2219#issuecomment-41804942
 python:
-  - "2.6"
-  - "2.7"
-virtualenv:
-  system_site_packages: true
+- "2.6"
+- "2.7_with_system_site_packages"
 script: PYTHONHASHSEED=random python setup.py test
 install:
   - sudo apt-get update