use image_size ~> 1.5 with apng detection, so apng images are not optimised to one frame version, partially resolves #142

Author: toy

image_optim.gemspec

@@ -21,7 +21,7 @@ You can safely remove `config.assets.image_optim = false` if you are not going t
 EOF
 
   s.add_dependency 'fspath', '~> 3.0'
-  s.add_dependency 'image_size', '~> 1.3'
+  s.add_dependency 'image_size', '~> 1.5'
   s.add_dependency 'exifr', '~> 1.2', '>= 1.2.2'
   s.add_dependency 'progress', '~> 3.0', '>= 3.0.1'
   s.add_dependency 'in_threads', '~> 1.3'