Don't override SSHKit.config.command_map

SSHKit's documentation says it's not wise to override it:

> One can also override the command map completely, this may not be wise,
> but it would be possible, for example:
>
> SSHKit.config.command_map = Hash.new do |hash, command|
>   hash[command] = "/usr/local/rbenv/shims/#{command}"
> end

https://github.com/leehambley/sshkit/blob/4c0694a96f445f27209cc56b576cd998c6e4b658/README.md#the-command-map

This PR has been applied to capistrano/chruby already:

capistrano/chruby#1