Remove ActiveSupport::Concern#prepended usage for Rails 6 (#662)

`ActiveSupport::Concern#prepended` is only available in Rails 6.1+.
Since we support `>= 5.2` we can't rely on the `prepended` callback in
`lib/stimulus_reflex/cable_readiness.rb`.

Since we can easily support Rails 5.2 and Rails 6.0 with this fix.
Otherwise we would need to make StimulusReflex depend on Rails 6.1+,
which seems rather dramatic.

Fixes #656

lib/stimulus_reflex/cable_readiness.rb

@@ -1,14 +1,8 @@
 # frozen_string_literal: true
 
-require "active_support/concern"
-
 module StimulusReflex
   module CableReadiness
-    extend ActiveSupport::Concern
-
-    prepended do
-      attr_reader :cable_ready
-    end
+    attr_reader :cable_ready
 
     def initialize(*args, **kwargs)
       super(*args, **kwargs)

test/cable_readiness_test.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require_relative "test_helper"
+
+class MyReflexPrepended
+  prepend StimulusReflex::CableReadiness
+
+  def id
+    "123"
+  end
+
+  def stream_name
+    "123"
+  end
+end
+
+class StimulusReflex::CableReadinessTest < ActiveSupport::TestCase
+  test "can be prepended" do
+    reflex = MyReflexPrepended.new
+    assert_includes reflex.methods, :cable_ready
+  end
+end