component/wkhtmltopdf: update download URL

component/wkhtmltopdf/buildout.cfg

 [buildout]
 
-extends = 
+extends =
   ../freetype/buildout.cfg
   ../fontconfig/buildout.cfg
   ../libpng/buildout.cfg
@@ -13,7 +13,7 @@ parts =
 
 [wkhtmltopdf]
 recipe = slapos.recipe.build
-url = http://download.gna.org/wkhtmltopdf/0.12/0.12.4/wkhtmltox-0.12.4_${:_url}.tar.xz
+url = https://github.com/wkhtmltopdf/wkhtmltopdf/releases/download/0.12.4/wkhtmltox-0.12.4_${:_url}.tar.xz
 environment =
   PATH=${xz-utils:location}/bin:%(PATH)s
 install =