Remove references to rgeos

[mikemahoney218]

re #207

[rsbivand]

Thanks. "SpatialCollections" are from rgeos too, though:

> getClass("Spatial")
Class "Spatial" [package "sp"]

Slots:
                              
Name:         bbox proj4string
Class:      matrix         CRS

Known Subclasses: 
Class "SpatialPoints", directly
Class "SpatialMultiPoints", directly
Class "SpatialGrid", directly
Class "SpatialLines", directly
Class "SpatialPolygons", directly
Class "SpatialPointsDataFrame", by class "SpatialPoints", distance 2
Class "SpatialPixels", by class "SpatialPoints", distance 2
Class "SpatialMultiPointsDataFrame", by class "SpatialMultiPoints", distance 2
Class "SpatialGridDataFrame", by class "SpatialGrid", distance 2
Class "SpatialLinesDataFrame", by class "SpatialLines", distance 2
Class "SpatialPixelsDataFrame", by class "SpatialPoints", distance 3
Class "SpatialPolygonsDataFrame", by class "SpatialPolygons", distance 2
> library(rgeos)
> getClass("SpatialCollections")
Class "SpatialCollections" [package "rgeos"]

Slots:
                                                                  
Name:             pointobj             lineobj             ringobj
Class:   SpatialPointsNULL    SpatialLinesNULL    SpatialRingsNULL
                                                                  
Name:              polyobj           plotOrder                bbox
Class: SpatialPolygonsNULL             integer              matrix
                          
Name:          proj4string
Class:                 CRS

Extends: "Spatial"