update platform_file / platform_url to reflect new release names (#29)
update platform_file / platform_url to reflect new release names (#29)

Starting with 20190523 release [1], Joyent_Dev names for the platform image
are now of the form

  `platform-release-$date-$fulliso8601`

This patch extracts the date and prepends `-release-$date` to the
platform filename make the download succeed.

[1] http://us-east.manta.joyent.com/Joyent_Dev/public/SmartOS/20190523T082605Z/index.html
calmh/smartos-platform-upgrade