Fix scoping error that caused stack trace

MSP-11489

When cracking an NTLM hash with an existing Login, Creation#create_cracked_credential would fail with: 
```
  NameError undefined local variable or method `core'
```

Note that this is a bugfix so only the `PATCH` version is bumped and thus Framework's Gemfile does not need to be updated.

# Verification
- [x] edit framework's gemfile to use this branch:

  ```
  gem 'metasploit-credential', git: 'https://github.com/rapid7/metasploit-credential.git', branch: 'bug/MSP-11489/cracked-scope'
  ```
- [x] Get an NTLM hash with an associated login.  If you don't already have one follow these steps:
  - [x] `use exploit/windows/smb/psexec`
  - [x] `set SMBUser msfadmin`
  - [x] `set SMBPass 1443d06412d8c0e6e72c57ef50f76a05:27c433245e4763d074d30a05aae0af2c`
  - [x] `set RHOST <host with msfadmin:msfadmin>`
  - [x] `run`
- [x] `use auxiliary/analyze/jtr_crack_fast`
- [x] `run`
- [x] **VERIFY** *before*: stack trace. *after*: successfully stores cracked credentials

# Post-merge Steps

Perform these steps prior to pushing to master or the build will be broke on master.

## Version
- [x] Edit `lib/metasploit/credential/version.rb`
- [x] Remove `PRERELEASE` and its comment as `PRERELEASE` is not defined on master.

## Gem build
- [x] gem build *.gemspec
- [x] VERIFY the gem has no '.pre' version suffix.

## RSpec
- [x] `rake spec`
- [x] VERIFY version examples pass without failures

## Commit & Push
- [ ] `git commit -a`
- [ ] `git push origin master`


# Release

## MRI Ruby
- [ ] `rvm use ruby-1.9.3@metasploit_data_models`
- [ ] `rm Gemfile.lock`
- [ ] `bundle install`
- [ ] `rake release`