Use children.any? since ObjectHierarchy is not

supported on MySQL

ee/app/models/ee/epic.rb

@@ -280,7 +280,7 @@ module EE
     end
 
     def has_children?
-      descendants.any?
+      children.any?
     end
 
     def has_issues?

ee/spec/models/epic_spec.rb

@@ -732,7 +732,7 @@ describe Epic do
     it 'has child epics' do
       create(:epic, group: group, parent: epic)
 
-      expect(epic.has_children?).to be_truthy
+      expect(epic.reload.has_children?).to be_truthy
     end
   end