Remove 2 defunct SBT download URLs and replace with the 1 known download URL. Also, use https.
Follow up on https://github.com/apache/spark/pull/7792

Author: Sean Owen <sowen@cloudera.com>

Closes #7956 from srowen/SPARK-9633 and squashes the following commits:

caa40bd [Sean Owen] Remove 2 defunct SBT download URLs and replace with the 1 known download URL. Also, use https.