pull latest version of Metasploit-Credential

fixes multi-threaded handling of uniqueness validations
around credential objects

MSP-11757

VERIFICATION STEPS 
- [x] LAND https://github.com/rapid7/metasploit-credential/pull/81
- [x] `bundle update metasploit-credential`
- [x] `rake spec`
- [x] VERIFY specs pass