build(deps-dev): bump rubocop-rails from 2.20.2 to 2.21.1

Bumps [rubocop-rails](https://github.com/rubocop/rubocop-rails) from 2.20.2 to 2.21.1.
- [Release notes](https://github.com/rubocop/rubocop-rails/releases)
- [Changelog](https://github.com/rubocop/rubocop-rails/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-rails@v2.20.2...v2.21.1)

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

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

Gemfile.lock

@@ -191,7 +191,7 @@ GEM
     mini_magick (4.12.0)
     mini_mime (1.1.5)
     mini_portile2 (2.8.4)
-    minitest (5.19.0)
+    minitest (5.20.0)
     msgpack (1.7.2)
     nested_form (0.3.2)
     net-imap (0.3.7)
@@ -317,7 +317,7 @@ GEM
     rubocop-performance (1.19.0)
       rubocop (>= 1.7.0, < 2.0)
       rubocop-ast (>= 0.4.0)
-    rubocop-rails (2.20.2)
+    rubocop-rails (2.21.1)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
       rubocop (>= 1.33.0, < 2.0)