Using exit code 1 of exec for positive virus scan

Hi,

I noticed that when I test the ClamAV exec with a positive file, that we would get an error.

It turns out that when ClamAV detects a virus, it returns an exit code of 1. Exec, by default, detects this as an error, while in this case it isn't. As a result the clamscan library also sees it as an error.  
This PR corrects the error handling to interpret err.code===1 as a positive virus match.

If you're wondering, we tested the detection of a virus by using [Eicar test file](http://www.eicar.org/86-0-Intended-use.html). When you create a file containing the 68 byte string given there, ClamAV (and other scanners) will see it as positive.

Thanks for making this library! It helped me a great deal. Hopefully this PR helps you in return :-)

Cheers,

Dieter