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] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -22,7 +22,7 @@ GEM
     docile (1.1.5)
     hashdiff (0.3.7)
     hashie (3.5.7)
-    json (2.1.0)
+    json (2.3.1)
     mustermann (1.1.1)
       ruby2_keywords (~> 0.0.1)
     omniauth (1.8.1)