Fix for bug in collapseTree

Not long after submitting a new version of phytools (phytools 0.5-38) to CRAN, which has been accepted, I discovered a weird bug in the interactive function collapseTree. collapseTree is interactive, so it is kind of hard to demo; however, a video of it's use can be seen here.

The bug is due to the plotrix function draw.circle either no longer being vectorized or not being correctly vectorized. I essentially fixed this by substituting:

draw.circle(x,y,radius=r,border=col,col="white",nv=20)

for:

nulo<-mapply(draw.circle,x=x,y=y,radius=r,MoreArgs=list(border=col,
    col="white",nv=20)

This fix is on GitHub and can be installed in the typical way.