Add minimum Ruby version

Author: mrcasals

date_validator.gemspec

@@ -20,6 +20,8 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rake', '>= 12.3.3'
   s.add_development_dependency 'tzinfo'
 
+  s.required_ruby_version = Gem::Requirement.new(">= 2.2".freeze)
+
   s.files         = `git ls-files`.split("\n")
   s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
   s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }