Include index.html page at db.apache.org/derby/releases/index.html

The folder at http://db.apache.org/derby/releases is not intended
to be navigated directly; the releases available for download are
managed via http://db.apache.org/derby/derby_downloads.html

We should include an index.html page at the raw releases directory
so that if a user accidentally visits that page they don't get an
auto-generated listing page from the underlying web server.

See: http://mail-archives.apache.org/mod_mbox/db-derby-dev/201602.mbox/%3C56B028F5.5040701%40gmail.com%3E
for additional discussion