Fix unit tests when linting with Swift 4.1

[marcelofabri]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Bug Report

Even after #2020, some unit tests are failing when a Swift 4.1 toolchain is used.

Executed 345 tests, with 1126 failures (0 unexpected) in 58.262 (58.485) seconds

Environment

• SwiftLint version (run swiftlint version to be sure)? master
• Installation method used (Homebrew, CocoaPods, building from source, etc)? source
• Which Xcode version are you using (check xcode-select -p)? 9.2 with swift-4.1-DEVELOPMENT-SNAPSHOT-2018-01-23-a.xctoolchain selected

[marcelofabri]

testDetectSwiftVersion is only failing because I'm compiling with Swift 4.0.3, but running with a 4.1 toolchain. It shouldn't happen after we start using Xcode 9.3.