Use geojson extension for data

[dannyfritz]

Github has built in features to aid with previewing geojson files.

[tomschenkjr]

Indeed, that feature was released after these data sets were published. Unfortunately, it doesn't support projections where we store data in our source systems using the X/Y coordinates of the Illinois State Plane system.

For the time-being, we've opted to maintain this repository with X/Y to maintain consistency between our source systems and the repo.