Update coverage badge URL
Update coverage badge URL

Seems like Coveralls switched up the badge URL on us for some reason.
loyso/tree-builder-builder,loyso/tree-builder-builder,dglazkov/tree-builder-builder,chunmun/tree-builder-builder,dglazkov/tree-builder-builder,chunmun/tree-builder-builder,loyso/tree-builder-builder,chunmun/tree-builder-builder,dglazkov/tree-builder-builder