Update index.js

Heya,

When trying to throw in a glob of files the output is not shown.

```js
gulp.task('jscs', function () {
    return gulp.src('./src/common/**/*.js')
        .pipe(jscs('./.jscsrc'));
});
```

Without the this.push(file). The functionality for a single file will still work.

But it will also output the error messages for multiple files and arrays of files

```js
return gulp.src([ 'file1.js',  'file2.js' ])
        .pipe(jscs('./.jscsrc'));
//and
    return gulp.src('./src/common/**/*.js')
        .pipe(jscs('./.jscsrc'));
```