Maintenance: Bump rubocop from 1.27.0 to 1.28.1

Bumps [rubocop](https://github.com/rubocop/rubocop) from 1.27.0 to 1.28.1.
- [Release notes](https://github.com/rubocop/rubocop/releases)
- [Changelog](https://github.com/rubocop/rubocop/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop/rubocop/compare/v1.27.0...v1.28.1)

.rubocop/default.yml

@@ -254,6 +254,10 @@ Style/RedundantBegin:
   Description: 'This cop checks for redundant `begin` blocks.'
   Enabled: false
 
+Style/FetchEnvVar:
+  Description: 'This cop suggests `ENV.fetch` for the replacement of `ENV[]`.'
+  Enabled: false
+
 Rails/EnvironmentVariableAccess:
   Description: 'This cop looks for direct access to environment variables through the ENV variable within the application code.'
   Enabled: true

Gemfile.lock

@@ -475,13 +475,13 @@ GEM
       rspec-core (> 3.3)
     rspec-support (3.11.0)
     rszr (1.2.1)
-    rubocop (1.27.0)
+    rubocop (1.28.1)
       parallel (~> 1.10)
       parser (>= 3.1.0.0)
       rainbow (>= 2.2.2, < 4.0)
       regexp_parser (>= 1.8, < 3.0)
       rexml
-      rubocop-ast (>= 1.16.0, < 2.0)
+      rubocop-ast (>= 1.17.0, < 2.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 3.0)
     rubocop-ast (1.17.0)