nimble_operator rule is not triggered for beTrue()

[Igor-Palaguta]

Describe the bug

nimble_operator rule is not triggered for beTrue() Nimble's predicate

Environment

• SwiftLint version (run swiftlint version to be sure)?
  0.30.1
• Installation method used (Homebrew, CocoaPods, building from source, etc)?
  package file from github
• Paste your configuration file:

opt_in_rules:
  - nimble_operator

• Which Xcode version are you using (check xcode-select -p)?
  Version 10.0 (10A255)
• Do you have a sample that shows the issue?

echo "expect(success).to(beTrue())" | swiftlint lint --no-cache --use-stdin --enable-all-rules

// This triggers a violation:
expect(success).to(beTrue())

// This does not trigger a violation:
expect(success) == true

[marcelofabri]

I think it was a conscious decision when I first wrote that rule because I personally preferred to use to(beTrue()) at that time.

But we should catch this though. PRs are welcome!

[marcelofabri]

Implemented in #2622.