version 0.3.4 seems incompatible with ImageMagick-6.4.3.6-7.30.1

[sockmonk]

After upgrading to simple-captcha2 0.3.4, I get this error:

StandardError (Error while running convert: convert: unrecognized evaluate operator Uniform-noise'. ): simple_captcha2 (0.3.4) lib/simple_captcha/utils.rb:17:inrun'
simple_captcha2 (0.3.4) lib/simple_captcha/image.rb:81:in generate_simple_captcha_image' simple_captcha2 (0.3.4) lib/simple_captcha/middleware.rb:42:inmake_image'
simple_captcha2 (0.3.4) lib/simple_captcha/middleware.rb:21:in `call'

That option doesn't appear to be available in this version:
% convert -list evaluate
Add
And
Divide
LeftShift
Log
Max
Min
Multiply
Or
Pow
RightShift
Set
Subtract
Xor

Could you either make it compatible with older versions, maybe by making newer options optional, or at least document the minimum version of ImageMagick required?