via not shown for packages installed from VCS

my `requirements.in` file has:

```
-e git+https://github.com/scikit-image/scikit-image.git@33646e0#egg=scikit-image==0.12.git
```

but the `.txt` just shows:

```
dask[array]==0.7.6
```

without showing the `via`. This even though I gave the package a name with the `egg=`.
