[Fix #461] Add new LoadDefaultsMismatch cop

[nzlaura]

Fixes #461

Introduces a new cop that checks that the version of rails in the config.load_defaults matches the version of Rails being used by a project. I have not allowed an autocorrect as it's not a safe change for the cop to make for you- instead this cop raises a warning that the version does not match.

I've followed the ruby style guide in implementing this, to check that the version in the config.load_defaults matches the exact version being used in the project.

https://rails.rubystyle.guide/#config-defaults

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.