Switch to maintained Paperclip fork

Recently, kt-paperclip 4.4.0 was released, so we can safely switch
to the maintained kt-paperclip fork while keeping the gem on the
same major version.

core/solidus_core.gemspec

@@ -39,7 +39,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'kaminari-activerecord', '~> 1.1'
   s.add_dependency 'mini_magick', '~> 4.10'
   s.add_dependency 'monetize', '~> 1.8'
-  s.add_dependency 'paperclip', '>= 4.2'
+  s.add_dependency 'kt-paperclip', '>= 4.4.0'
   s.add_dependency 'paranoia', '~> 2.4'
   s.add_dependency 'ransack', '~> 2.0'
   s.add_dependency 'state_machines-activerecord', '~> 0.6'