One alternative upscaling method (for non-integer ratios) is to first upscale with no interpolation a integer ratio to make a bigger image than the target size. Then the large image is down-scaled (with some suitable algorithm/filtering/interpolation) to final size. Afaik it is what Apple does with it's "Retina" MacBooks and thus I have dubbed it "retina scaling", but there is probably a proper name for the method too.

edit: comparison http://imgur.com/a/fC8iQ#1