Force MDM to 0.17.4 as 0.17.5 breaks things

This is breaking new source-based installations with the following error:
```
ruby-1.9.3-p547@metasploit-framework/bundler/gems/metasploit_data_models-0b1fc5baf50c/app/models/metasploit_data_models/search/operator/multitext.rb:5:in `<top (required)>': uninitialized constant MetasploitDataModels::Search::Operator (NameError)
```

Fix MDM or merge this, either way.