fix _ssh_start_args method. invalid keyname :host changes valid keyname ...

fix _ssh_start_args method. 
because key name ":host" is not valid for Net::SSH.
fix key name ":host" to "host_name"

in .. /vendor/bundle/ruby/2.0.0/gems/net-ssh-2.9.0/lib/net/ssh.rb
> VALID_OPTIONS = [:auth_methods, :bind_address, :compression, :compression_level, :config, :encryption, :forward_agent, :hmac, :host_key, :keepalive, :keepalive_interval, :kex, :keys, :key_data, :languages, :logger, :paranoid, :password, :port, :proxy, :rekey_blocks_limit, :rekey_limit, :rekey_packet_limit, :timeout, :verbose, :global_known_hosts_file, :user_known_hosts_file, :host_key_alias, :host_name, :user, :properties, :passphrase, :keys_only, :max_pkt_size, :max_win_size, :send_env, :use_agent]