Update INDEX.md

I would remove any mention of Joomla! 1.5 support. Furthermore, it doesn't appear the Framework is available for download in 1.5. At least I couldn't find it here: https://code.google.com/p/gantry-framework/downloads/list?can=1&q=platform%3Djoomla25&colspec=Filename%20Summary%20Uploaded%20Size%20DownloadCount%20Type%20Platform&num=100&start=100