cannot make build-ghcjs due to missing servant-jsaddle directory

`make build-ghcjs` fails due to missing servant-jsaddle directory.

```
$ make build-ghcjs
cabal v2-build --builddir=dist-newstyle-ghcjs --project-file=cabal.ghcjs.project all
When using configuration(s) from /Users/heath/local/servant/cabal.ghcjs.project, the following errors occurred:
The package location 'servant-jsaddle/' does not exist.

make: *** [build-ghcjs] Error 1
```