fix `options.rootDir` problem

When I don't define `optoins.rootDir` in options, such as:

```js
gulp.task('jest', function () {
  var nodeModules = path.resolve('./node_modules');

  return gulp.src('app/scripts/**/__tests__')
    .pipe($.jest({
        scriptPreprocessor: nodeModules + '/gulp-jest/preprocessor.js',
        unmockedModulePathPatterns: [nodeModules + '/react']
    }));
});
```

When in `scripts` dir, It have more than one `__tests__` dir, such as `scripts/dropdown/__tests__` and `scripts/nav/__tests__`, the `options.rootDir`
 always be `scripts/dropdown/__tests__`, and will test `scripts/dropdown/__tests__` twice。

![gulp-jest](http://cookfront.qiniudn.com/gulp-jest.png)

I think `gulp.src` have defined test dir,  and need not define `options.rootDir`, So I think set the `options.rootDir` always be `file.path`

```js
options.rootDir = file.path;
```