Exception "ValueError: License type is missing" shouldn't be raised

Hello,

```
datapackage.DataPackage("https://raw.githubusercontent.com/datasets/gold-prices/master/")
```

raises

```
ValueError: License type is missing
```

maybe we should expect to have set it to `None` and not raises an exception
(just send a warning using `warnings.warn`)
because we can't do anything if an exception is raised

Kind regards
