Fix gem version to support rubygems < 2.1

https://jira.tor.rapid7.com/browse/MSP-10714

## Verification Steps

- [ ] `gem update --system 1.8.23`
- [ ] `bundle`
- [ ] VERIFY: bundle completes successfully
- [ ] VERIFY: `rake spec` completes successfully

## Land
- [ ] Merge this PR
- [ ] Remove `PRERELEASE` constant and comment from `lib/metasploit/credential/version.rb`.  Commit and push.