Interactive Churn of a Binary File Must Be Undefined

# Description

The _interactive churn_ metric of a binary file must be undefined. However, the implementation of the metric incorrectly returns `0.0`.

## Demonstration

```
$ nameko shell
>>> n.rpc.interactivechurn.collect(n.rpc.project.get('google', 'brotli'), '534072ad426a566dbe54f767686088f282ef4ffc', 'docs/brotli-comparison-study-2015-09-22.pdf')
0.0
```