Duplicate vertex ids when adding high number of vertices

When adding large number of vertices, some of my vertices weren't added to the graph.

For example
```
vertices = 0..250000 |> Enum.map(& &1)
Graph.add_vertices(Graph.new, vertices)
#Graph
```

I fixed it for myself by changing the function `Graph.Utils.vertex_id(v)` to
``` def vertex_id(v), do: v```
instead of
``` def vertex_id(v), do: :erlang.phash2(v, @max_phash)```

Apparently I had duplicate key in my graph otherwise.