script to create initial version.php file

This may not be the best way to go about it fixing it, but a freshly-cloned instance of p5js.org produces several warnings because there is no initial download/version.php. Running git-clone.php also results in an error because there is no site/ directory by default. This script does everything needed to eliminate the warnings I have seen so far, except for the warning addressed in https://github.com/lmccart/p5js.org/pull/68