Stripping y1="0" on a linearGradient confuses some browsers

Per the SVG 1.1 spec, on a `linearGradient`, the default value of `y1` is 0. svgo is therefore correct to strip such an attribute.

Unfortunately, however, some browser engines (I tested recent-ish versions of Firefox and Chromium) seem to think the default value of `y1` should be 1, not 0. As a result, optimized SVGs with such a gradient will render wrong in these browsers.

This trivial change just makes svgo not strip a default `y1` from a `linearGradient`, thus avoiding the problem.