snapRbind uses defunct function rBind

[imran-aifi]

I am getting this error when running Step 1 of the PBMC tutorial: https://github.com/r3fang/SnapATAC/tree/master/examples/10X_PBMC_15K#barcode_selection :

> x.sp = Reduce(snapRbind, x.sp.ls);
Error: 'rBind' is defunct; 'base::rbind' handles S4 objects since R 3.2.0
> traceback()
5: stop(errorCondition(msg, old = fname, new = new, package = package, 
       class = "defunctError"))
4: .Defunct(msg = "'rBind' is defunct; 'base::rbind' handles S4 objects since R 3.2.0")
3: Matrix::rBind(bmat1, bmat2)
2: f(init, x[[i]])
1: Reduce(snapRbind, x.sp.ls)

This seems like a simple patch to update rBind to base::rbind.