[BUGFIX] Shows download button only for secure version

If the latest version of an extension is insecure
the download button has to disappear.

Change-Id: I91cb822060d7f716f60aba0526d16885bb3a98a8
Resolves: #61545
Reviewed-on: http://review.typo3.org/33989
Tested-by: Thomas Löffler <loeffler@spooner-web.de>
Reviewed-by: Tomas Norre Mikkelsen <tomasnorre@gmail.com>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>