🎉 Support ActiveAdmin 2.x and code style improvements

- Add support ActiveAdmin for 2.x
- Add rubocop config
- Add frozen_string_literal to ruby files
- Apply changes to respect rubocop alerts

activeadmin_quill_editor.gemspec

@@ -1,4 +1,6 @@
-lib = File.expand_path('../lib', __FILE__)
+# frozen_string_literal: true
+
+lib = File.expand_path('lib', __dir__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require 'activeadmin/quill_editor/version'
 
@@ -15,5 +17,5 @@ Gem::Specification.new do |spec|
   spec.files         = `git ls-files -z`.split("\x0")
   spec.require_paths = ['lib']
 
-  spec.add_runtime_dependency 'activeadmin', '~> 1.0'
+  spec.add_runtime_dependency 'activeadmin', '>= 1.0'
 end