TopoJson.get_bounds() returns [lon,lat] instead of [lat,lon]

`TopoJson._get_self_bounds()` returns `[[xmin, ymin], [xmax, ymax]]` i.e. a bounding box defined by longitude,latitude points, rather than lat,lon points.

This is contrary to `GeoJson._get_self_bounds()` as well as contrary to the expectations of `FitBounds`:

> bounds: list of (latitude, longitude) points
