Fix Node >= 4.8
Fix Node >= 4.8

Node 4.8 supports the shell option for spawn now, update cross-spawn to
respect that.

bradleylandis/testem,testem/testem,johanneswuerbach/testem,bradleylandis/testem,testem/testem,bradleylandis/testem,testem/testem,johanneswuerbach/testem,johanneswuerbach/testem