Don't raise an error if BeautifulSoup isn't installed

In the docs it says BeautifulSoup is an optional dependency:
http://django-compressor.readthedocs.org/en/latest/quickstart/#dependencies

I came across others having the issue of 'Error while importing lxml: No module named BeautifulSoup' while using Python 2:
https://github.com/jezdez/django_compressor/issues/261

This tiny patch lets you carry on without installing BeautifulSoup, just like Python 3 users, instead of raising an error. It makes it possible to just use lxml.