Fix gem version to support rubygems < 2.1

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