Cannot raco pkg install

I was hoping I could just `raco pkg install formica` but that gives an error:

```
Resolving "formica" via https://download.racket-lang.org/releases/6.6/catalog/
Resolving "formica" via https://pkgs.racket-lang.org
raco pkg install: cannot use empty checksum for Git repostory package source
  source: github://github.com/samsergey/formica/blob/master/
```
