use https instead of http for schema.org link

[mkoohafkan]

I recently switched to https for my Github-pages site, and I'm seeing a mixed-content warning on (some) browsers. I checked the page source and the only plain http reference I see is to http://schema.org, which I think is generated by the jekyll-seo-tag gem. Any chance you can update the gem to use https://schema.org instead?

[pathawks]

Good idea. This is fully supported by schema.org and, like you say, might be good for us to adopt.

Care to open a pull request?

[DirtyF]

Fixed in #296