Update rubocop version to 0.57.x (#7078)

Merge pull request 7078

.rubocop.yml

@@ -98,6 +98,8 @@ Security/YAMLLoad:
   Exclude:
     - !ruby/regexp /features\/.*.rb/
     - !ruby/regexp /test\/.*.rb$/
+Style/AccessModifierDeclarations:
+  Enabled: false
 Style/Alias:
   EnforcedStyle: prefer_alias_method
 Style/AndOr:

Gemfile

@@ -25,7 +25,7 @@ group :test do
   gem "nokogiri", "~> 1.7"
   gem "rspec"
   gem "rspec-mocks"
-  gem "rubocop", "~> 0.56.0"
+  gem "rubocop", "~> 0.57.2"
   gem "test-dependency-theme", :path => File.expand_path("test/fixtures/test-dependency-theme", __dir__)
   gem "test-theme", :path => File.expand_path("test/fixtures/test-theme", __dir__)
 

lib/jekyll/drops/drop.rb

@@ -15,11 +15,7 @@ module Jekyll
       #
       # Returns the mutability of the class
       def self.mutable(is_mutable = nil)
-        @is_mutable = if is_mutable
-                        is_mutable
-                      else
-                        false
-                      end
+        @is_mutable = is_mutable || false
       end
 
       def self.mutable?