Specify help as the default target
Specify help as the default target

This is already happening implicitly, since it's the first target
listed. Assuming this is the desired behavior, I think we should make it
explicit so it's not displaced by a newer target that someone happens to
place above `help`.

Signed-off-by: Shon Feder <ea1e9e869ba967b9537eb0b16fe47f610d2cc997@gmail.com>

dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder