how to configured shell_api parameter in carrierwave with mini magic

My project use the carrierwave 0.9.0 with mini_magick 4.2.10 so I want to know how to configured the shell_api parameter in mini_magick using carrierwave.

MiniMagick.configure do |config|
  config.shell_api = "posix-spawn"
end

Where should I write this configuration.