output dir cannot contain directories

```js
concat: {
    outputs: [{
        file: '/concat.js',
        inputs: ['file1.js', 'file2.js']
    }]
},
files: ['/concat.js']
```
results in:
```
ERROR [preprocess]: Can not load "concat"!
  Error: ENOENT: no such file or directory, open '/project//concat.js'
```

My goal was to use `require('os').tmpdir()`.