Handle removal of CacheClass

Django removed django.core.cache.backends.memcached.CacheClass
https://github.com/django/django/commit/59351247bd635c55bb72a156d71f0d3a27563a67

When using memcache as the cache backend, I have to catch ImportError to make djcelery work with the Django development version (parentheses around this tuple are used to catch both).