Fix joinroot method
Fix joinroot method

jessamynsmith/fontbakery,googlefonts/fontbakery,graphicore/fontbakery,davelab6/fontbakery,graphicore/fontbakery,moyogo/fontbakery,googlefonts/fontbakery,graphicore/fontbakery,moyogo/fontbakery,googlefonts/fontbakery,moyogo/fontbakery