Add support for != comparison to Rails/UnknownEnv

[lovro-bikic]

Expands Rails/UnknownEnv so it registers offenses for unknown environment names in inequality comparisons:

Rails.env != 'proudction'
'proudction' != Rails.env

At the time of writing, there are ~1.4k search results for this pattern on GitHub.

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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.

[koic]

Given the nature of a linter, it would make the changelig entry easier to understand if the text began with either "Fix false positives" or "Fix false negatives", whichever is appropriate.

[lovro-bikic]

Updated

[koic]

Thanks!