Revert to sane detection code

Author: gerdint

frog/responsive-images.rkt

@@ -21,7 +21,7 @@
 (define identify (find-executable-path "identify"))
 (define convert (find-executable-path "convert"))
 
-(define magick-available? (find-executable-path convert))
+(define magick-available? (and identify convert))
 
 (module+ test
   (require rackunit))