style: resolve rubocop issues

Gemfile

@@ -1,6 +1,8 @@
+# frozen_string_literal: true
+
 source 'https://rubygems.org'
 
-gem 'rake'
 gem 'coffee-script'
-gem 'uglifier'
+gem 'rake'
 gem 'sass'
+gem 'uglifier'

Rakefile

@@ -1,25 +1,29 @@
+# frozen_string_literal: true
+
 require 'bundler'
 
 Bundler.setup
 
 require 'uglifier'
 
-desc "Compile"
+desc 'Compile'
 task :compile do
-  Dir.glob("**/*.coffee").each do |file|
+  Dir.glob('**/*.coffee').each do |file|
     system("bundle exec coffee -c #{file}")
   end
 
-  Dir.glob("**/*.scss").each do |file|
-    css_name = file.sub(/\.scss/, ".css")
+  Dir.glob('**/*.scss').each do |file|
+    css_name = file.sub(/\.scss/, '.css')
     system("bundle exec sass --scss #{file} > #{css_name}")
   end
 
-  File.open('chardinjs.min.js', 'w') { |f| f.write Uglifier.compile(File.read('chardinjs.js')) }
+  File.open('chardinjs.min.js', 'w') do |f|
+    f.write Uglifier.compile(File.read('chardinjs.js'))
+  end
 end
 
-desc "Watch"
+desc 'Watch'
 task :watch do
-  files = Dir.glob("**/*.coffee").join(' ')
+  files = Dir.glob('**/*.coffee').join(' ')
   system("bundle exec coffee -wc #{files}")
 end