Fix wrong detection when run in background in shell script

Without this change, this shell script:

```shell
#!/bin/bash
nodemon foo &
```

results in the following warning:

```
30 Nov 20:14:15 - [nodemon] watching: /tmp
30 Nov 20:14:15 - [nodemon] Unable to write to temp directory. If you experience problems with file reloading, ensure /tmp is writable.
```

In a shell script in the background, I see `watchFile` getting the value 0 (first fd available - won't have a stdin) - which is after all a valid fd.

(Ubuntu 13.10)