def incr(key, amt = 1, ttl = nil, default = nil)
      check_positive!(amt)

      perform(:incr, key, amt.to_i, ttl_or_default(ttl), default)
    end
#
Incr adds the given amount to the counter on the memcached server.
Amt must be a positive integer value.

If default is nil, the counter must already exist or the operation
will fail and will return nil.  Otherwise this method will return
the new value for the counter.

Note that the ttl will only apply if the counter does not already
exist.  To increase an existing counter and update its TTL, use
#cas.

If the value already exists, it must have been set with raw: true