Copy pyspark and SparkR packages to latest release dir too

## What changes were proposed in this pull request?

Copy pyspark and SparkR packages to latest release dir, as per comment [here](https://github.com/apache/spark/pull/16226#discussion_r91664822

)

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16227 from felixcheung/pyrftp.

(cherry picked from commit c074c96d)
Signed-off-by: Shivaram Venkataraman <shivaram@cs.berkeley.edu>