Build a compressed DMG rather than read-write.

I noticed that the plugins get distributed as a read/write DMG for OS X, which is non-standard. Normally people distribute DMGs as compressed read-only.

`hdiutil` actually creates compressed, read-only DMGs by default, but in this case I specified it explicitly with the `UDZO` option rather than `UDRW`. One side benefit is that the output DMG is about a sixth the size compared to the read/write image.

Thanks for your consideration!