You still have the wrong mathjax plugin installed.
Change git clone https://github.com/kasperpeulen/discourse-mathjax.git to
git clone https://github.com/discourse/discourse-math.git
You still have the wrong mathjax plugin installed.
Change git clone https://github.com/kasperpeulen/discourse-mathjax.git to
git clone https://github.com/discourse/discourse-math.git