Bump rubocop-rails from 2.15.2 to 2.21.2

Bumps [rubocop-rails](https://github.com/rubocop/rubocop-rails) from 2.15.2 to 2.21.2.
- [Release notes](https://github.com/rubocop/rubocop-rails/releases)
- [Changelog](https://github.com/rubocop/rubocop-rails/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-rails@v2.15.2...v2.21.2)

---
updated-dependencies:
- dependency-name: rubocop-rails
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -90,10 +90,10 @@ GEM
     rubocop-performance (1.19.1)
       rubocop (>= 1.7.0, < 2.0)
       rubocop-ast (>= 0.4.0)
-    rubocop-rails (2.15.2)
+    rubocop-rails (2.21.2)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
-      rubocop (>= 1.7.0, < 2.0)
+      rubocop (>= 1.33.0, < 2.0)
     ruby-progressbar (1.13.0)
     tzinfo (2.0.6)
       concurrent-ruby (~> 1.0)