43 gulp concat with sourcemaps bug

issue: #43 

The last update of gulp-concat, which supports sourcemaps isn't working properly. Below you have the output:

```bash
./node_modules/.bin/gulp
[13:07:59] Using gulpfile ~/boards-ui/gulpfile.js
[13:07:59] Starting 'buildScripts'...
[13:07:59] Starting 'buildStyles'...
[13:07:59] Starting 'watch'...
[13:07:59] Finished 'watch' after 14 ms
gulp-sourcemap: source file not found:/Users/catalinm/boards-ui/src/styles/clearfix.less
gulp-sourcemap: source file not found:/Users/catalinm/boards-ui/src/styles/headings.less
gulp-sourcemap: source file not found:/Users/catalinm/boards-ui/src/styles/hello-world.less
gulp-sourcemap: source file not found:/Users/catalinm/boards-ui/src/styles/box-shadow.less
gulp-sourcemap: source file not found:/Users/catalinm/boards-ui/src/styles/typography.less
```

`gulpfile.js` snippet:

```javascript
gulp.task('buildStyles', function(){
  return gulp.src(paths.build.styles)
    .pipe(plumber())
    .pipe(sourcemaps.init())
    .pipe(concat('boards.less'))
    .pipe(less())
    .pipe(sourcemaps.write('./maps'))
    .pipe(gulp.dest(paths.build.outputPath));
});
```


The fix is quite easy, file.path wasn't mentioned right