Downloads page should say Mac requires 10.8 Mountain Lion

[PaulStoffregen]

Please update the downloads page to say 10.8 Mountain Lion is required for Macintosh.

Arduino 1.8.6 crashes when run on 10.7. The hourly build and future versions will refuse to run, due to #7957.

[facchinm]

Fixed and pushed on website, thanks 😉