Notice here how we have to add another library (D3 in this case, but could be jQuery) to load the external GeoJSON file.
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forked from enjalot's block: WWSD #1-b: Leaflet starter
Modified http://cdn.leafletjs.com/leaflet/v0.7.7/leaflet.js to a secure url
https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js
https://cdn.leafletjs.com/leaflet/v0.7.7/leaflet.js