Update profile images to use the new Twitter API

The Twitter API v1.0 has been officially retired and is no
longer active: https://dev.twitter.com/blog/api-v1-is-retired.
Add `map.geojson`

Take advantage of GitHub's geographic visualization feature by adding a
`.geojson` file containing the location where the meetup is being held:
https://github.com/blog/1528-there-s-a-map-for-that.