Unique Metasploit::Credential::Cores

[MSP-10238](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&view=detail&selectedIssue=MSP-10238)

# Verification Steps

- [x] `bundle install`

## `rake spec`
- [x] `rake spec`
- [x] VERIFY no failures

# Post-merge Steps

Perform these steps prior to pushing to master or the build will be broke on master.

## Version
- [x] Edit `lib/metasploit/credential/version.rb`
- [ ] Remove `PRERELEASE` and its comment as `PRERELEASE` is not defined on master.

## Gem build
- [ ] gem build *.gemspec
- [ ] VERIFY the gem has no '.pre' version suffix.

## RSpec
- [ ] `rake spec`
- [ ] VERIFY version examples pass without failures

## Commit & Push
- [ ] `git commit -a`
- [ ] `git push origin master`

# Release

Complete these steps on DESTINATION

## `VERSION`

`MINOR` has already been incremented and `PATCH` reset to 0 because the addition of a validation and unique index is an incompatible change.

## JRuby
- [ ] `rvm use jruby@metasploit_data_models`
- [ ] `rm Gemfile.lock`
- [ ] `bundle install`
- [ ] `rake release`

## MRI Ruby
- [ ] `rvm use ruby-1.9.3@metasploit_data_models`
- [ ] `rm Gemfile.lock`
- [ ] `bundle install`
- [ ] `rake release`