COMP: Download jqplot source package from slicer.kitware.com

Since github.com deprecated its download api [1], the package
is now downloaded from slicer.kitware.com.

[1] https://github.com/blog/1302-goodbye-uploads

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21684 3bd1e089-480b-0410-8dfb-8563597acbee