Semantic Versioning

[MSP-11157](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&selectedIssue=MSP-11157)

# Pre-verification

* Merge https://github.com/rapid7/metasploit-yard/pull/1

# Verification

## specs
- [ ] `rake spec`
- [ ] VERIFY no failures

## README

- [ ] VERIFY README doesn't have any errors.
- [ ] VERIFY README has no left over, metasploit-credential documentation.

## CONTRIBUTING

- [ ] VERIFY CONTRIBUTING has no errors.
- [ ] VERIFY CONTRIBUTING does not have have references to other gems left over from copying the file.