Update Go crawler to support the new download site.

Latest downloads are now on golang.org; older versions remain on Google Code.
[FIXED JENKINS-23509] Write Go download URLs which don't point to 302 redirects.
[JENKINS-22237] Gradle available versions should be in descending order.
[FIXED JENKINS-24329] Update Go crawler to handle download site changes.

The download page now provides direct, absolute URLs and we're now more lenient
about picking up links from the page, as the filename regex will handle things.
Allow selection of specific JDK 8, not just the latest.
I shouldn't edit after I've tested...
[FIXED JENKINS-23826] Allow automatic install of Maven 3.2.2