Make geojson-validation an optional or dev dependency

[adamfranco]

Many users of OpenMapSamples won't actually be writing fresh GeoJSON to feed into SimpleSampleData and will use other SampleData classes or pre-validated GeoJSON.

Making geojson-validation an optional or dev dependency would reduce the dependency chain for downstream consumers of this library.

See https://docs.npmjs.com/cli/v8/commands/npm-install 's section on the --save-optional flag.