FIX Issue 966

Ruby 1.9.3 no longer had .id method for the Object class. Changed calls for
repository files objects and repository directory objects (which inherited it from Object) to use
object_id instead because it is still available in 1.9.3

This will allow assignment submission while using ruby version 1.9.3.

Tested using rails 3.0.18 and ruby 1.9.3.