add postinst script to remove hmac-sha1 from sshd_config on existing instances
add postinst script to remove hmac-sha1 from sshd_config on existing instances

conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,heartsucker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,heartsucker/securedrop,conorsch/securedrop