GitHub hosted css files not loading anymore in ioslides

Question

After a recent update of knitr, my ioslides don't seem to load the CSS file hosted on github. This used to work fine but now it doesn't anymore.

Locally the CSS file works fine, and the css file hosted on Github is publicly accessible.

output :
  ioslides_presentation:
    logo           : https://shklinkenberg.github.io/uva_style/images/logo_uva.png
    smaller        : FALSE
    css            : https://shklinkenberg.github.io/uva_style/ioslides_style.css
    self_contained : TRUE
    widescreen     : TRUE

Previously the CSS would be used for knitting the ioslides. I am running Rstudio on a mac.