Hi,
this is a common problem, in my template I also have 2 or 3 plugins which use external sources (forecasts etc.) and if those pages are not HTTPS then there is nothing you can do and these plugins only work for HTTP people. It is a bit unfortunate that those who use the more advanced and no doubt better solution (https) are currently still a bit discriminated in this respect...
It is a bit of a viscous cycle, since to solve this you would need more and more pages to be https, on the other hand more people dont want to update if they use external sources because it would be a problem. I am 100% sure that over time it will be part of every webhosting as well and so in a few year's time everyone will be running HTTPS, but it will take some time.