Semantic versioning 2 0

[MSP-10011](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&view=detail&selectedIssue=MSP-10011)

Update metasploit-model to use the version.rb and version_spec.rb that checks that Rapid7 gems are following semantic versioning 2.0 and use the branch name for PRERELEASE.  Include description of this versioning in the README too.