Update gemspec with minimum ruby/rails versions

Author: Stanislav Katkov

sprockets-rails.gemspec

@@ -11,12 +11,12 @@ Gem::Specification.new do |s|
 
   s.files = Dir["README.md", "lib/**/*.rb", "MIT-LICENSE"]
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.5'
 
   s.add_dependency "sprockets", ">= 3.0.0"
-  s.add_dependency "actionpack", ">= 4.0"
-  s.add_dependency "activesupport", ">= 4.0"
-  s.add_development_dependency "railties", ">= 4.0"
+  s.add_dependency "actionpack", ">= 5.2"
+  s.add_dependency "activesupport", ">= 5.2"
+  s.add_development_dependency "railties", ">= 5.2"
   s.add_development_dependency "rake"
   s.add_development_dependency "sass"
   s.add_development_dependency "uglifier"