Can't build PDF

I try to build PDF like following and I got ENOENT error and there is no pdf.

```
$ gitbook pdf ./GITBOOK/
Starting build ...
ENOENT, lstat '/var/folders/hr/qd9blw7d2qxg40xj8cj9wb6m0000gn/T/tmp-8381liwbiww/index.pdf'
```

When try to build with 

```
gitbook build ./GITBOOK/ --format=pdf
```

I found index.pdf. However, I can't open this pdf file.
