Log Message:
textproc/ruby-json-pure: update to 2.3.1

Update ruby-json-pure package to 2.3.1.

## 2020-06-30 (2.3.1)

* Spelling and grammar fixes for comments. Pull request #191 by Josh
  Kline.
* Enhance generic JSON and #generate docs. Pull request #347 by Victor
  Shepelev.
* Add :nodoc: for GeneratorMethods. Pull request #349 by Victor Shepelev.
* Baseline changes to help (JRuby) development. Pull request #371 by Karol
  Bucek.
* Add metadata for rubygems.org. Pull request #379 by Alexandre ZANNI.
* Remove invalid JSON.generate description from JSON module rdoc. Pull
  request #384 by Jeremy Evans.
* Test with TruffleRuby in CI. Pull request #402 by Benoit Daloze.
* Rdoc enhancements. Pull request #413 by Burdette Lamar.
* Fixtures/ are not being tested... Pull request #416 by Marc-André
  Lafortune.
* Use frozen string for hash key. Pull request #420 by Marc-André
  Lafortune.
* Added :call-seq: to RDoc for some methods. Pull request #422 by Burdette
  Lamar.
* Small typo fix. Pull request #423 by Marc-André Lafortune.