fix: extend compatibility to rubies 2.6 and 2.5

The `Date::Error` exception class was added with Ruby 2.7 but this gem still supports a couple of versions below that.

lib/itax_code/parser.rb

@@ -84,7 +84,7 @@ def day
 
       def birthdate
         @birthdate ||= Date.parse("#{year}-#{month}-#{day}").to_s
-      rescue Date::Error
+      rescue ArgumentError
         raise DateTaxCodeError
       end