Relax Ruby required version in Gemspec

Author: filippoliverani

solidus_auth_devise.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |s|
     s.metadata["source_code_uri"] = s.homepage if s.homepage
   end
 
-  s.required_ruby_version = '~> 2.4'
+  s.required_ruby_version = '>= 2.4.0'
 
   s.files = Dir.chdir(File.expand_path(__dir__)) do
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }