Directory size deviates from what `du -sh` computes

The [`directorySize`](https://github.com/klaussner/meteor-cleaner/blob/1619d28e3de232cc2fe134c92366356159c295a5/lib/directory-size.js) function returns a smaller value than `du -sh`. On macOS, it seems to deviate more than on Linux.

**Linux example:**
```
meteor-cleaner: 9.98 GiB
du -sh: 10 GiB
```

**macOS example:**
```
meteor-cleaner: 3.74 GiB
du -sh: 5.1 GiB
```
