If the removePattern method is throwing that error then memcached is not configured correctly in your factories.yml configuration.
The answer you want is discussed on this page under the heading 'Alternative Caching storage'
You need to set your class and parameter correctly to use the removePattern method. For instance:
view_cache:
class: sfMemcacheCache
param:
host: localhost
port: 11211
persistent: true
storeCacheInfo: true
You will also need to insure that the memcached daemon is operational. If memcached is running on the same host as the script in it's default configuration then the above settings should be fine. If you are running memcached on a separate host or you have multiple hosts sharing a memcached service then you must insure your host, port and firewall settings are correct for your environment.