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

@@ -124,7 +124,7 @@ GEM
       railties (>= 3.2.16)
     jslint (1.2.0)
       json
-    json (2.1.0)
+    json (2.3.1)
     kaminari (1.2.1)
       activesupport (>= 4.1.0)
       kaminari-actionview (= 1.2.1)