Update Elm Platform to 0.13

The recommended uninstall method is to run an [external script](https://github.com/elm-lang/elm-platform/blob/master/src/mac/helper-scripts/uninstall.sh), which to my knowledge is not included in the distribution.

We do not currently support external resources in the uninstall phase, but pkg-based uninstall works acceptably well, leaving behind a single orphaned folder in `/usr/share`, which we remove with `:delete`.