Description of problem

Tackle 2.x allows users to add cyclic dependencies to applications. This issue doesn't happen in Tackle 1.2.

How reproducible

Always

Steps to Reproduce:

1.Create 3 applications: north, app, south
2. Add 'north' as a northbound dependency for app
3. Add 'south' as a southbound dependency for app
4. Add 'south' as a northbound dependency for 'north'

Actual results

Tackle allows cyclic dependencies

Expected results

Tackle shouldn't allow cyclic dependencies

Additional info