use latest credential gem

this fixes issues with post modules trying
to set a Login's status back to UNTRIED
after it has already been tried

MSP-11782

VERIFICATION STEPS
- [ ] LAND https://github.com/rapid7/metasploit-credential/pull/82
- [ ] `bundle update metasploit-credential`
- [ ] VERIFY now on 0.13.7
- [ ] `rake spec`
- [ ] VERIFY no failures