CRAN version 0.4.2

rmapshaper 0.4.2

• Added rgdal to Suggests so ms_clip and ms_erase can transform
  Spatial* objects when they have different CRSs.
• When an input sf object is a tibble, the output is now now also a tibble,
  (#95, thanks @mdsumner)
• Upgraded to mapshaper v0.4.107
• Bumped minimum version of V8 to 3.0