buildMix: fix bootstrapping packages with hyphens in version

I'm using `buildMix` to build a package, which has a dependency with a hyphen in its version: `dialyxir-1.0.0-rc6`.
Due to `mix-bootstrap` using `string:tokens` the dependency ends up in `_build/prod/lib/dyalyxir1.0.0rc6` instead of `_build/prod/lib/dialyxir`.
Here I'm fixing it to use `string:split`, which splits by first hyphen only, and returns an array with no more than two elements.