Work around incomplete libedit readline API in 5.3
Work around incomplete libedit readline API in 5.3

Fixes #164
mubassirhayat/psysh,damiankloip/psysh,lvht/psysh,adetch/psysh,mubassirhayat/psysh,damiankloip/psysh,damiankloip/psysh,bobthecow/psysh,adetch/psysh,lvht/psysh,bobthecow/psysh