Increase required ruby version to 2.3 (#1495)

Usage of the safe operator was introduced in ruby 2.3 and new code uses that

devise_token_auth.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |s|
   s.test_files = Dir['test/**/*']
   s.test_files.reject! { |file| file.match(/[.log|.sqlite3]$/) }
 
-  s.required_ruby_version = ">= 2.2.0"
+  s.required_ruby_version = ">= 2.3.0"
 
   s.add_dependency 'rails', '>= 4.2.0', '< 6.2'
   s.add_dependency 'devise', '> 3.5.2', '< 5'