Lock down rubocop development dependencies

Newer versions gives offenses, causing CI to fail. We'll address that in
a later PR.

Author: bquorning

.rubocop.yml

@@ -1,5 +1,6 @@
-require:
+plugins:
   - rubocop-rspec
+  - rubocop-sequel
 
 AllCops:
   TargetRubyVersion: 2.7

Gemfile

@@ -6,7 +6,8 @@ gemspec
 
 gem 'rake', '~> 13.2.1'
 gem 'rspec', '~> 3.7'
-gem 'rubocop-rspec', '~> 3.5'
+gem 'rubocop', '~> 1.76.0'
+gem 'rubocop-rspec', '~> 3.5.0'
 gem 'sequel', '~> 5.47'
 gem 'simplecov', '~> 0.16'
 gem 'sqlite3', '~> 1.3', '>= 1.3.12'