Updated devel/ocaml-jbuilder to version 1.0+beta19.1.

The biggest change is that the package is now called 'dune' rather than
'jbuilder'; I'm waiting until the package comes out of beta to apply that
name change to pkgsrc.

Otherwise there have been mostly incremental changes and bugfixes,
nothing major; for more information see CHANGES.md.

(jaapb)