Fix for engine requires

MSP-11412

## Verification steps

- [ ] `rake spec`


## Post-merge steps

- [ ] Remove the `PRERELEASE` line from `version.rb`
- [ ] Remove the line for `metasploit-concern` in the `Gemfile`
- [ ] Remove the line for `metasploit-model` in the `Gemfile`
- [ ] Remove the comment from following line in the `gemspec`:

```ruby
s.add_runtime_dependency 'metasploit-concern'#, '~> 0.3.0'
```

- [ ] Remove the comment from following line in the `gemspec`:

```ruby
s.add_runtime_dependency 'metasploit-model'#,'~> 0.28.0'
```
        
- [ ] `bundle`
- [ ] Commit and push to master
- [ ] Make a new RubyGems release
Ah, I didn't see the Post-merge step to remove this.  Nevermind.  =)