Update cms_plugins.py

when using python 3 I had an issue with xrange. I beleive it should work with Python 2 as well ?
issue http://stackoverflow.com/questions/25106591/cant-add-columns-in-multicolumns-for-django-cms
https://github.com/divio/djangocms-column/issues/14
Merge pull request #17 from zen-coder/patch-1

Update cms_plugins.py