Increment version to signal incompatibility in 0.17.4+ versions

[MSP-10654](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&view=detail&selectedIssue=MSP-10654)

@hololeap (in https://github.com/rapid7/metasploit_data_models/issues/65) and @hmoore-r7 both brought up that 0.17.4 is actually incompatible with rapid7/metasploit-framework/master, so increment the version to 0.18.0 to signal incompatibility.

This also removed the dead code for non-`Rails::Engine` loading, such as `MetasploitDataModels.require_models` to more fully-signal the incompatibility in the API.