Bug/msp 11592/blank password failure

update gemspec to use newest metasploit-credential

MSP-11592

VERIFICATION STEPS 
- [x] land https://github.com/rapid7/metasploit-credential/pull/78
- [x] switch to this branch
- [x] `bundle update metasploit-credential`
- [x] `rake spec`
- [x] verify all specs pass