Added editable concern spec

spec/models/concerns/editable_spec.rb

+require 'spec_helper'
+
+describe Editable do
+  describe '#is_edited?' do
+    let(:issue) { create(:issue, last_edited_at: nil) }
+    let(:edited_issue) { create(:issue, created_at: 3.days.ago, last_edited_at: 2.days.ago) }
+
+    it { expect(issue.is_edited?).to eq(false) }
+    it { expect(edited_issue.is_edited?).to eq(true) }
+  end
+end