Fix mixed content

This makes `https://betravis.github.io/icon-methods/*` available over HTTPS.

Before this patch, on e.g. <https://betravis.github.io/icon-methods/svg-sprite-sheets.html>:

> Mixed Content: The page at 'https://betravis.github.io/icon-methods/svg-sprite-sheets.html' was loaded over HTTPS, but requested an insecure stylesheet 'http://fonts.googleapis.com/css?family=Source+Code+Pro'. This request has been blocked; the content must be served over HTTPS.