Invalid tryKill result if EPERM error catched

shell:
```
$ ps -p 37
  PID TTY           TIME CMD
   37 ??         0:04.41 /usr/sbin/KernelEventAgent
```

node cli:
```
> var ir = require('./is-running')
undefined
> ir(37)
false
> process.kill(37,0)
Error: kill EPERM
...
```

Fixed via error code check for `"EPERM"` value.