Relax the ImageJ 1.x requirement of Sholl Analysis

ImagePlus#deleteRoi() was introduced in 1.46m, and
ImageProcessor#getBitDepth() in 1.47a. Both calls can be replaced
easily with calls supported back to ImageJ 1.29, though, so let's
do that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>