Lock the version of virtualenv.

The latest version of virtualenv install pip 1.5 which uses distlib which expects
a home directory where it can create a '.distlib' directory.

See: https://bitbucket.org/pypa/distlib/issue/39/using-distlib-with-pip-fails-when-usre-has

playbooks/roles/common/defaults/main.yml

@@ -37,7 +37,7 @@ common_debian_pkgs:
   - python2.7-dev
 
 common_pip_pkgs:
-  - virtualenv
+  - virtualenv==1.10.1
   - virtualenvwrapper
 
 common_web_user: www-data