Okbuck fails if exportDependencies > file path doesn't exist

Example config

```
exportDependencies {
        enabled = true
        file = "build/raw-deps.json"
}
```

The folder build do not exist. okbuck throws the following exception

```
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':okbuck'.
> java.nio.file.NoSuchFileException: /home/user/my_project/build/raw-deps.json
```