Add .jar extension to mock version to download it properly

Change-Id: Ic1fb0a14082836e5d78b6edb0b92f87fe24b2678
Reviewed-on: http://review.couchbase.org/28204
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>