Enforce compatible core version

see: https://www.openproject.org/work_packages/13947
Merge pull request #104 from finnlabs/fix/enforce_right_core_version

Enforce compatible core version
Add spec
Fix missing story points
Merge pull request #107 from finnlabs/hotfix/show_story_points_in_wp_show
Adapt version to corresponding OpenProject core version
Merge branch 'release/3.0' into dev

Conflicts:
	lib/open_project/backlogs/engine.rb
	lib/open_project/backlogs/version.rb
	spec/controllers/work_packages_controller_spec.rb