Fixes: Rails/UniqueValidationWithoutIndex

Currently validates :column_name, uniqueness: false will report an offence if the schema does not specify a unique index for the corresponding database column. The cop is currently insensitive to the value of the :uniqueness option. Any value (including falsy false and nil) will cause this cop to expect a unique index in the schema.

This pull request fixes this behaviour such that we will now only report offences when the value of the :uniqueness option is truthy.

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.