Bump json from 2.1.0 to 2.3.1

Bumps [json](https://github.com/flori/json) from 2.1.0 to 2.3.1.
- [Release notes](https://github.com/flori/json/releases)
- [Changelog](https://github.com/flori/json/blob/master/CHANGES.md)
- [Commits](ruby/json@v2.1.0...v2.3.1)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -27,7 +27,7 @@ GEM
       concurrent-ruby (~> 1.0)
     irb (1.0.0)
     jaro_winkler (1.5.2)
-    json (2.1.0)
+    json (2.3.1)
     kramdown (1.17.0)
     mini_portile2 (2.4.0)
     minitest (5.11.3)