fix race condition in create credential core

now that core has a uniqueness constraint we have to close
up the race condition.

VERIFICATION STEPS
- [ ] point pro to this branch by using path in the gemfile
- [ ] start pro
- [ ] create a new project with the following in the network range:
```
creds-2k3sql-01.ms.scanlab.rapid7.com
creds-centos-01.ms.scanlab.rapid7.com
creds-ub1204-01.ms.scanlab.rapid7.com
creds-w28-mssql.ms.scanlab.rapid7.com
creds-w7apps-01.ms.scanlab.rapid7.com
```
- [ ] do discovery scan
- [ ] select all hosts and run psexec
- [ ] use msfadmin:msfadmin
- [ ] set payload to bind
- [ ] run exploit
- [ ] VERIFY you do not get a stack trace with: 
`Exploit failed: ActiveRecord::RecordNotUnique PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_metasploit_credential_privates_on_type_and_data"
DETAIL:  Key (type, data)=(Metasploit::Credential::Password, notpassword) already exists.`

# Release

Complete these steps on DESTINATION

## `VERSION`

### Compatible changes

If your change are compatible with the previous branch's API, then increment [`PATCH`](lib/metasploit/credential/version.rb).

### Incompatible changes

If your changes are incompatible with the previous branch's API, then increment [`MINOR`](lib/metasploit/credential/version.rb) and reset [`PATCH`](lib/metasploit/credential/version.rb) to `0`.

- [ ] Following the rules for [semantic versioning 2.0](http://semver.org/spec/v2.0.0.html), update [`MINOR`](lib/metasploit/credential/version.rb) and [`PATCH`](lib/metasploit/credential/version.rb) and commit the changes.

## 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`