make sure sourcemap has file property

Recently vinyl-sourcemaps-apply (0.1.4) introduced a fix to make sure the source map contains `.file` property. Otherwise it would throw an error saying 

```
Source map to be applied is missing the "file" property
...
```

The PR also fixes the .css extension.
Merge pull request #3 from madhums/patch-1

Thanks much for the PR, sorry I missed this for so long. :+1: