def required_string(name)
      required_custom(name) do |env|
        if value = env[name.to_s.upcase]
          {name => value}
        else
          raise ConfigError.new("A value must be provided for #{name.to_s.upcase}")
        end
      end
    end
def required_string(name)
      required_custom(name) do |env|
        if value = env[name.to_s.upcase]
          {name => value}
        else
          raise ConfigError.new("A value must be provided for #{name.to_s.upcase}")
        end
      end
    end
Define a required string config var.