Make checklist plugin compatible to Redmine 4.0.4

[lammel]

This pull request will make the Checklist Plugin compatible with Redmine 4.0/Rails 5.x.

Tested with Redmine 4.0.4 and Ruby 2.5.5p157 (Debian).

[lammel]

This pull request might also resolve/help with issue #33

[lammel]

The TravisCI build failures do not seem to be related to the changes at all.
So the Travis setup should be fixed too.

[adrianobr]

Working 4.1

[lammel]

Yes, it is compatible with Redmine 4.1 too. Using it already.
Merging would be nice.

[YongHwan-Jeong]

Redmine 4.0.6 migration error.

Is there a solution?

---

rake aborted!
StandardError: An error has occurred, all later migrations canceled:

Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for:

class CreateIssueChecklists < ActiveRecord::Migration[4.2]

[tony1223]

@YongHwan-Jeong it's related the rails version, you could use my PR.