Release 2.8.0

Fixes for:

• Changing class names on SVG nodes where the node's class was tainted by being animated (and those being a SVGAnimatedString. From tsayen#250 by https://github.com/eddydg
• Error fetching the file extension when a resource URL has a query string that has a . character in it. From tsayen#285 by https://github.com/CG-man

Bump dev dependency on mocha to version 6.1.4 to fix vulnerability warning