Add support for geometry rings

[ajturner]

esri-dump https://services5.arcgis.com/8CMfd3uYk8d4J22E/ArcGIS/rest/services/VDHFishConAdvAGOL/FeatureServer/0 > vdh_Fish.geojsonn
/Users/andr7059/.nvm/versions/node/v11.10.1/lib/node_modules/esri-dump/lib/geometry.js:107
                geometry: rings2geojson(feature.geometry.rings)
                                                         ^

TypeError: Cannot read property 'rings' of undefined
    at toGeoJSON (/Users/andr7059/.nvm/versions/node/v11.10.1/lib/node_modules/esri-dump/lib/geometry.js:107:58)
    at /Users/andr7059/.nvm/versions/node/v11.10.1/lib/node_modules/esri-dump/lib/geometry.js:84:26
    at Array.forEach (<anonymous>)
    at Request._callback (/Users/andr7059/.nvm/versions/node/v11.10.1/lib/node_modules/esri-dump/lib/geometry.js:80:27)
    at Request.self.callback (/Users/andr7059/.nvm/versions/node/v11.10.1/lib/node_modules/esri-dump/node_modules/request/request.js:185:22)
    at Request.emit (events.js:197:13)
    at Request.<anonymous> (/Users/andr7059/.nvm/versions/node/v11.10.1/lib/node_modules/esri-dump/node_modules/request/request.js:1154:10)
    at Request.emit (events.js:197:13)
    at IncomingMessage.<anonymous> (/Users/andr7059/.nvm/versions/node/v11.10.1/lib/node_modules/esri-dump/node_modules/request/request.js:1076:12)
    at Object.onceWrapper (events.js:285:13)