Bump json from 2.7.6 to 2.8.1

Bumps [json](https://github.com/ruby/json) from 2.7.6 to 2.8.1.
- [Release notes](https://github.com/ruby/json/releases)
- [Changelog](https://github.com/ruby/json/blob/master/CHANGES.md)
- [Commits](ruby/json@v2.7.6...v2.8.1)

---
updated-dependencies:
- dependency-name: json
  dependency-type: indirect
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -159,7 +159,7 @@ GEM
       rails-dom-testing (>= 1, < 3)
       railties (>= 4.2.0)
       thor (>= 0.14, < 2.0)
-    json (2.7.6)
+    json (2.8.1)
     logger (1.6.1)
     loofah (2.23.1)
       crass (~> 1.0.2)