#2346: Error when expiring block caches

Description of the bug

After creating/editing/removing articles, Noosfero tries to expire cache of some blocks and raises an error.

Noosfero is using regexp to expire caches, but it is not supported by memcached.

From http://apidock.com/rails/v2.3.2/ActionController/Caching/Fragments/expire_fragment:

Regexp - Will remove any fragment that matches, so %r{pages/\d*/notes} might remove all notes.
Make sure you don't use anchors in the regex (^ or $) because the actual filename matched looks
like ./cache/filename/path.cache. Note: Regexp expiration is only supported on caches that can 
iterate over all keys (unlike memcached). 

-- DanielaFeitosa -- 21 Apr 2012