fix missing invalidation of the cached resolved DI object in InstanceRequirements
models: get rid of Port#actual_name

This is really an artifact of the old composition port handling.

It fixes an inconsistency between Composition#port_mappings_for
and composition models (closes #12)
Merge pull request #14 from rock-core/fix_issue12

models: get rid of Port#actual_name
Merge remote-tracking branch 'autobuild/master' into realtime

Conflicts:
	lib/syskit/instance_requirements.rb