this makes this work again with gulp.dest(...)

*Warning* i am new to gulp / (through|pipe|event-stream) paradigm.  But i believe that gulp-bench has a regression with gulp.dest(...)

So doing 
```js
gulp.src('my-test.js', {read: false}).
    pipe(benchmark()).
    pipe(gulp.dest('.'));
```
wont work unless `this.push(f)` is present.

I am using gulp `3.8.8`