Update traitlets requirement
Update traitlets requirement

@scorlay
ChakriCherukuri/bqplot,bloomberg/bqplot,ssunkara1/bqplot,dmadeka/bqplot,dmadeka/bqplot,ChakriCherukuri/bqplot,ChakriCherukuri/bqplot,bloomberg/bqplot,SylvainCorlay/bqplot,SylvainCorlay/bqplot,SylvainCorlay/bqplot,ssunkara1/bqplot,bloomberg/bqplot