[BUG] Cannot pipe another tasks at end

I'm trying to do:

```javascript
return gulp.src('./copy-src/**/*.ts')
    .pipe(compodoc({
        output: './docs/',
        tsconfig: './tsconfig.json',
        theme: 'readthedocs',
        serve: false
    }))
    .pipe(del('./copy-src/'));
```

or anything else pipe task after, hasn't been called
