deprecated functions public_key.ssh_decode/2 and public_key.ssh_encode/2 in configurable_client_keys.ex

[Jbalcek]

mix compile
==> sshex
Compiling 3 files (.ex)
warning: :public_key.ssh_decode/2 is undefined or private, use ssh_file:decode/2 instead
Invalid call found at 2 locations:
  lib/sshex/configurable_client_keys.ex:41: SSHEx.ConfigurableClientKeys.add_host_key/3
  lib/sshex/configurable_client_keys.ex:61: SSHEx.ConfigurableClientKeys.is_host_key/4

warning: :public_key.ssh_encode/2 is undefined or private, use ssh_file:encode/2 instead
  lib/sshex/configurable_client_keys.ex:43: SSHEx.ConfigurableClientKeys.add_host_key/3

public_key.ssh_decode/2 and public_key.ssh_encode/2 are now deprecated.