fixed a bug in prelude-create-scratch-buffer

Two updates in `prelude-create-scratch-buffer`.
1. Use `funcall` to set scratch buffer major mode.  Previous implementation does not get along with `global-linum-mode` somehow.
2. Simplify `(get-buffer-create (generate-new-buffer-name "*scratch*"))` with just `(generate-new-buffer "*scratch*")`
@bbatsov oops, my fault. should be ready to go now.