iPhone 5s has the same density as 5 and 4

The iPhone 5s (hw.machine values of iPhone6,1 and iPhone6,2) has the
same pixel density as the iPhone 5.

This fixes issue #106.

See also:
https://github.com/SVGKit/SVGKit/pull/111
https://github.com/SVGKit/SVGKit/issues/106
Merge pull request #1 from cbhl/master

iPhone 5S has the same density as 5 and 4