Add cc-by 4.0 for geojson files

[bolkedebruin]

Makes sure correct attribution is there for geojson files.

See also for "cc-by 4.0" evidencing: http://journals.plos.org/plosone/article/file?type=supplementary&id=info:doi/10.1371/journal.pone.0151586.s002

cc: @mistercrunch @kristw

[codecov-io]

Codecov Report

Merging #6731 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6731   +/-   ##
=======================================
  Coverage   56.14%   56.14%           
=======================================
  Files         521      521           
  Lines       23179    23179           
  Branches     2769     2769           
=======================================
  Hits        13014    13014           
  Misses       9756     9756           
  Partials      409      409

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[mistercrunch]

LGTM