Phylo.draw_ascii: ensure tree is rendered the same in Py and Py3

Python 3 uses "banker's" rounding, i.e. round(0.5) -> 0, round(1.5) -> 2;
Python 2 uses "sane" rounding, i.e. round(0.5) -> 1.
Result: the little stub used to draw the root did not appear on Python 3.
This commit fixes that, at the cost of some fidelity to the actual tree or
Phylo.draw() image.
Merge branch 'master' of github.com:biopython/biopython