Fix the Memcached::Rails fetch method to work when no block is given.

Fix Memcached::Rails fetch method to behave like ActiveSupport's fetch method.  The current fetch method returns an error if no block is given.  According to the documentation for fetch:

```ruby
    # Fetches data from the cache, using the given key. If there is data in
    # the cache with the given key, then that data is returned.
    #
    # If there is no such data in the cache (a cache miss), then nil will be
    # returned. However, if a block has been passed, that block will be run
    # in the event of a cache miss. The return value of the block will be
    # written to the cache under the given cache key, and that return value
    # will be returned.
```