Log Message:
textproc/ruby-json-pure: update to 2.5.1

# Changes

## 2020-12-22 (2.5.1)

* Restore the compatibility for constants of JSON class.

## 2020-12-22 (2.5.0)

* Ready to Ractor-safe at Ruby 3.0.

## 2020-12-17 (2.4.1)

* Restore version.rb with 2.4.1

## 2020-12-15 (2.4.0)

* Implement a freeze: parser option #447
* Fix an issue with generate_pretty and empty objects in the Ruby and Java
  implementations #449
* Fix JSON.load_file doc #448
* Fix pure parser with unclosed arrays / objects #425
* bundle the LICENSE file in the gem #444
* Add an option to escape forward slash character #405
* RDoc for JSON #439 #446 #442 #434 #433 #430