[0.13.7] Fixes json serialization turning license into {}.

While trying to reproduce #1639 with 0.13.7's cached resolution, I ran into several issues:
- minigraph (json4s serialization) persists license `(Some("name of license"), None)` into `{}`, which is unserializable.
- cached resolution in scripted test bleeding out to global location.

The temporary workaround for the first issue is to put `Some("http://localhost")` in place for `None`.
For the second issue I introduced `dependencyCacheDirectory` key, which is set to 

```scala
dependencyCacheDirectory := (baseDirectory in LocalRootProject).value / "dependency"
```

in scripted tests.

/review @jsuereth 

so license url is non-optional?