CRAN version 0.4.0

New features

• Added sys argument to allow the use of the system mapshaper if it's installed (#61)

Improvements and bug fixes

• Upgraded to mapshaper v0.4.64 (#60)
• sf::st_read() is now used throughout for reading from disk and from geojson
  strings, which allows for greater consistency and better performance.
• Better handling of different column classes (#68, thanks @mdsumner)
• Avoid stackoverflow caused by adding special geojson classes (#71,
  ropensci/geojsonio#128)
• The name of the sf column is now properly retained (#70)
• Fixed issue where encoding/special characters were not preserved (#67)