fix: revert required_ruby_version to >=2.4 for 2.1.2

Closes #228

Author: bufferoverflow

lib/omniauth-saml/version.rb

@@ -1,5 +1,5 @@
 module OmniAuth
   module SAML
-    VERSION = '2.1.1'
+    VERSION = '2.1.2'
   end
 end

omniauth-saml.gemspec

@@ -10,10 +10,10 @@ Gem::Specification.new do |gem|
   gem.authors       = ['Raecoo Cao', 'Ryan Wilcox', 'Rajiv Aaron Manglani', 'Steven Anderson', 'Nikos Dimitrakopoulos', 'Rudolf Vriend', 'Bruno Pedro']
   gem.homepage      = 'https://github.com/omniauth/omniauth-saml'
 
-  gem.required_ruby_version = '>= 3.1'
+  gem.required_ruby_version = '>= 2.4'
 
   gem.add_runtime_dependency 'omniauth', '~> 2.1'
-  gem.add_runtime_dependency 'ruby-saml', '>= 1.17'
+  gem.add_runtime_dependency 'ruby-saml', '~> 1.17'
 
   gem.add_development_dependency 'rake', '~> 13.2'
   gem.add_development_dependency 'rspec', '~> 3.13'