move to librsvg instead of rsvg

I offer to switch dependency rsvg to my fork of node-rsvg — [librsvg](https://github.com/anru/node-rsvg). Original node-rsvg doesn't accept my pull request walling/node-rsvg#4 more than six months. Also I fix problem when node-rsvg renders zero-sized buffer  of normal svg image in rare but real cases.