Link to version 7 in readme is broken

[hankhank10]

Hi Matt,

I just spent a good hour trying to work out why my code wasn't working properly before working out that it was a breaking change at the iodine side from the latest version. That'll teach me not to explicitly include version numbers in my CDN links...

In any case, the link to version 7 in your readme delivers a 404 so would be worth updating that:

[mattkingshott]

Thanks. Fixed it :)