Drop Ruby 2.4 support

Follow up to utkarsh2102@6972a97
and rubocop/rubocop#9648.

.rubocop.yml

@@ -9,7 +9,7 @@ AllCops:
     - 'spec/fixtures/**/*'
     - 'tmp/**/*'
     - 'vendor/**/*'
-  TargetRubyVersion: 2.4
+  TargetRubyVersion: 2.5
 
 Naming/FileName:
   Exclude:

rubocop-packaging.gemspec

@@ -21,12 +21,12 @@ Gem::Specification.new do |spec|
   spec.files         = Dir["config/default.yml", "lib/**/*", "LICENSE", "README.md"]
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.4.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.5.0")
 
   spec.add_development_dependency   "bump", "~> 0.8"
   spec.add_development_dependency   "pry", "~> 0.13"
   spec.add_development_dependency   "rake", "~> 13.0"
   spec.add_development_dependency   "rspec", "~> 3.0"
   spec.add_development_dependency   "yard", "~> 0.9"
-  spec.add_runtime_dependency       "rubocop", ">= 0.89", "< 2.0"
+  spec.add_runtime_dependency       "rubocop", ">= 1.13", "< 2.0"
 end

spec/rubocop/packaging_spec.rb

@@ -50,8 +50,7 @@
       end
     end
 
-    it "has a SupportedStyles for all EnforcedStyle " \
-      "and EnforcedStyle is valid" do
+    it "has a SupportedStyles for all EnforcedStyle and EnforcedStyle is valid" do
       errors = []
       cop_names.each do |name|
         enforced_styles = config[name]

tasks/cops_documentation.rake

@@ -25,8 +25,7 @@ task verify_cops_documentation: :generate_cops_documentation do
     # Output diff before raising error
     sh("GIT_PAGER=cat git diff docs")
 
-    warn "The docs directory is out of sync. " \
-      "Run `rake generate_cops_documentation` and commit the results."
+    warn "The docs directory is out of sync. Run `rake generate_cops_documentation` and commit the results."
     exit!
   end
 end