Updated regexp for filter system collections

Fixed cleaning collections with names like:

favourite_systems
operating_systems
systems

and so on.

All real mongo's system collections have following name format (as in 2.4.1):

  {db_name}.system.{system_collection_type}

lib/database_cleaner/moped/truncation_base.rb

@@ -23,7 +23,7 @@ module DatabaseCleaner
           session.use(db)
         end
 
-        session['system.namespaces'].find(:name => { '$not' => /system|\$/ }).to_a.map do |collection|
+        session['system.namespaces'].find(:name => { '$not' => /\.system\.|\$/ }).to_a.map do |collection|
           _, name = collection['name'].split('.', 2)
           name
         end