Fix mini_magick version to >= 4.9.4, due to CVE-2019-13574 vulnerability

Changelog.md

+## 0.3.0 [unreleased]
+* Fix mini_magick version to >= 4.9.4, due to CVE-2019-13574 vulnerability
+
+
 ## 0.2.0 [29 December 2016]
 * introduce support for Rails 4.2 and Refinery 3.0
 * introduce Strong Parameters protection instead of attr_*

refinerycms-photo-gallery.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |s|
   s.add_dependency    'refinerycms-core',     '~> 3.0.5'
   s.add_dependency    'mime-types'
   s.add_dependency    'carrierwave', '0.6.2'
-  s.add_dependency    'mini_magick'
+  s.add_dependency    'mini_magick', ">= 4.9.4"
   s.add_dependency    'mini_exiftool'
 
   # Development dependencies