[BUGFIX beta] Remove import statement of Router

Since the router.js sync in cf0caa04f625067ec3b4629acdec12c8517df94e,
a `TransitionAborted` rejection is handled by the router (see
http://git.io/Sf38aQ) so there is no need to re-check for this in
the Ember.Test.adapter#exception handler.
Merge pull request #3704 from pangratz/fix/import_of_router_module

[BUGFIX beta] Fix import of Router module in ember-testing package