Bump json from 1.8.6 to 2.3.1

Bumps [json](https://github.com/flori/json) from 1.8.6 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@v1.8.6...v2.3.1)

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

Author: dependabot[bot]

Gemfile.lock

@@ -126,7 +126,7 @@ GEM
       rails-dom-testing (>= 1, < 3)
       railties (>= 4.2.0)
       thor (>= 0.14, < 2.0)
-    json (1.8.6)
+    json (2.3.1)
     koala (3.0.0)
       addressable
       faraday