Merge pull request #3 from spark-solutions/fix-rails-version-check

use Gem::Version object to compare rails version instead of string

lib/acts_as_taggable_on/taggable/core.rb

@@ -50,7 +50,7 @@ def #{tag_type}_list=(new_tags)
               parsed_new_list = ActsAsTaggableOn.default_parser.new(new_tags).parse
 
               if self.class.preserve_tag_order? || parsed_new_list.sort != #{tag_type}_list.sort
-                if Rails.version < "6.0.0.beta1"
+                if Gem.loaded_specs["rails"].version < Gem::Version.new("6.0.0.beta1")
                   set_attribute_was('#{tag_type}_list', #{tag_type}_list)
                 else
                   duplicated_mutations_from_database.change_to_attribute('#{tag_type}_list')