Copy previous releases vendors by default

I have added the option (shown with default value)

```ruby
set :composer_copy_previous_vendors, true
```

Along with the task `composer:copy_from_previous_release` (the name sucks). It runs just before `composer:install` and if there is no change in dependencies between releases then it essentially makes `composer:install` a no-op, speeding things up somewhat (and saving on bandwidth, if there is no composer cache).