Use bufio.Reader instead of bufio.Scanner for logger.Copier

When using a scanner, log lines over 64K will crash the Copier with
bufio.ErrTooLong. Subsequently, the ioutils.bufReader will grow without
bound as the logs are no longer being flushed to disk.

Signed-off-by: Burke Libbey <burke.libbey@shopify.com>