Bump version to 1.40.0

[bquorning]

Please double check:

• Did I miss any new process re. VersionAdded/VersionChanged?
• Do we have any PRs that should be merged before cutting a release?

Changelog since last release:

• Add new RSpec/VariableName cop. (@tejasbubane)
• Add new RSpec/VariableDefinition cop. (@tejasbubane)
• Expand Capybara/VisibilityMatcher to support more than just have_selector. (@twalpole)
• Add new SpecSuffixOnly option to RSpec/FilePath cop. (@zdennis)
• Allow RSpec/RepeatedExampleGroupBody to differ only by described_class. (@robotdana)
• Fix RSpec/FilePath detection across sibling directories. (@rolfschmidt)
• Substantial performance inprovement of RSpec/SubjectStub. (@andrykonchin)

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[bquorning]

Rebased after #928.

[pirj]

All good.

Pushed a small amendment for FilePath since we've introduced a config option there. I believe it's completely optional since we didn't change the defaults though. We'll figure out the exact procedure as we go.

On the other hand, it might be of interest for those who update to be notified about those new options. Or may be a burden. Not sure yet.

[Darhazer]

When new options are introduced, even if default are not changed, we should bump the VersionChanged. At some point we could point the users to the new options, so they can tweak their configurations. For now I think we only point them to newly added cops.

[bquorning]

Thank you, both.