Metasploit::Credential::Core association search

[MSP-10029](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&view=detail&selectedIssue=MSP-10029)

Use `metasploit-credential` that supports searching associations on `Metasploit::Credential::Core` for https://github.com/rapid7/pro compatibility.

# Pre-Verification Steps

- [x] Release https://github.com/rapid7/metasploit-credential/pull/52
- [x] Update the Gemfile to use the released version of metasploit-credential

# Verification Steps

- [x] `bundle install`

## `rake spec`
- [x] `rake spec`
- [x] VERIFY no failures

# Post merge
- [ ] Merge https://github.com/rapid7/pro/pull/1407