SPARK-2462.  Make Vector.apply public.

Apologies if there's an already-discussed reason I missed for why this doesn't make sense.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1389 from sryza/sandy-spark-2462 and squashes the following commits:

2e5e201 [Sandy Ryza] SPARK-2462.  Make Vector.apply public.