Update Prepros item on install page for new version 5

Prepros has a new website (https://prepros.io/) and it is now also available for Linux.

Also, v5 isn't open source anymore, though v4 will remain open source (source: https://github.com/Subash/Prepros/issues/663#issuecomment-57082986). Deleted 'Open Source' in description because there is no mention of the repository on the Prepros website so new users are unlikely to encounter it.

I was not entirely sure about this one so I didn't include it, but should the Sass website include a mention that there is a free version of Prepros available?