Update CHANGELOG for beta 18
Update CHANGELOG for beta 18

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