Replace ggmap with ggspatial

[jack-davison]

{ggmap} requires an API key now, {ggspatial} does not.

Other benefits:

• Allows us to bring in the new crs updates done on the leaflet maps.

• More available tilesets (e.g., CARTO)

• Users can define their own coordinate limits with coord_sf()

Also fixes issues w/ modern {ggplot2}.

Fixes #52

[davidcarslaw]

Some good changes that will hep future development, thanks!