Error using _gen

Followed the [readme](https://github.com/go-dash/slice)
Installed gen
`brew install go-dash/tools/gen`

got slice
`go get github.com/go-dash/slice`


Tried to generated the code, got this error: 
```
_gen

ERROR: package github.com/go-dash/slice: module github.com/go-dash/slice@latest found (v0.0.0-20180716194816-31b150132b4d), but does not contain package github.com/go-dash/slice
```