Add lambda to the validations that use model constants

In this way when we need modify the constants model value in the
model/custom folder, adding lambda it will be possible load the new
values.

spec/models/progress_bar_spec.rb

@@ -64,6 +64,18 @@ describe ProgressBar do
     expect(progress_bar).to be_valid
   end
 
+  it "dynamically validates the percentage range" do
+    stub_const("#{ProgressBar}::RANGE", (-99..99))
+
+    progress_bar.percentage = -99
+
+    expect(progress_bar).to be_valid
+
+    progress_bar.percentage = 100
+
+    expect(progress_bar).not_to be_valid
+  end
+
   it "is not valid without a progressable" do
     progress_bar.progressable = nil