Add vector-effect attribute

[paulathevalley]

Ideally the grid stroke widths will stay fixed for responsive SVGs. There is support for retaining the stroke width using vector-effect="non-scaling-stroke", see: https://www.w3.org/TR/SVGTiny12/painting.html#NonScalingStroke

The vector-effect SVG attribute isn’t supported until React v15, whomp whomp.

Here’s a demo of a responsive chart with a non-scaling-stroke (text scaling looks OK as it grows, but as it shrinks there's a point when it's unreadable).

[boygirl]

@stefvhuynh if you have time you can drop this in along with your React 15 update