Change rack version to >= 1.6.11 due to CVE-2018-16471 vulnerability

Changelog.md

 ## 0.3.0 [unreleased]
 * Fix mini_magick version to >= 4.9.4, due to CVE-2019-13574 vulnerability
+* Change rack version to >= 1.6.11 due to CVE-2018-16471 vulnerability
 
 
 ## 0.2.0 [29 December 2016]

refinerycms-photo-gallery.gemspec

@@ -23,6 +23,7 @@ Gem::Specification.new do |s|
   s.add_dependency    'carrierwave', '0.6.2'
   s.add_dependency    'mini_magick', ">= 4.9.4"
   s.add_dependency    'mini_exiftool'
+  s.add_dependency    'rack', ">= 1.6.11"
 
   # Development dependencies
   s.add_development_dependency 'refinerycms-testing', '~> 3.0.5'