Offline pack cannot force and update to a dependencies

Installing a pack to upgrade existing plugins or a pack that come with an already installed dependency will fails.

**Scenario**
- update all the plugins on a local installation (LS 5.5.1)
- pack a plugin like `logstash-input-mongodb`
- the pack will come with an upgraded version of `stud`
- trying to install the pack in a freshly extract LS 5.5.1 will complaint that stud is already locked to a previous version.

**Debug message**
```
The bundle currently has stud locked at 0.0.22.
Try running `bundle update stud`
```

**Solution**

The problem is when we add [the new dependencies](https://github.com/elastic/logstash/blob/master/lib/pluginmanager/bundler/logstash_injector.rb#L82) to the Bundler's gem definition we never mark the gem as unlockable.