Prevents AppRouter._processOnRoute() passing "undefined" to _.invert() when appRoutes is configured only via constructor.

If appRoutes is configured when AppRouter is extended, "this.appRoutes" is defined.
If appRoutes is configured via constructor, "this.appRoutes" is undefined (held instead at "this.options.appRoutes").

AppRouter._processOnRoute() passes "this.appRoutes" to _.invert(), which throws a TypeError when undefined.

Fixed by replacing "this.appRoutes" with "this.getOption('appRoutes')", which searches "this.options" and "this".