.swift-version says it need Swift 5.0 although SwiftLint require Swift 5.1 in 0.40.0

[cfiken]

New Issue Checklist

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

Describe the bug

SwiftLint needs Swift 5.1 to build from 0.40.0 Release.
However, .swift-version is still specified as Swift 5.0, which means that it will be built with swift 5.0 and will fail when you are using swiftenv.

Complete output when running SwiftLint, including the stack trace and command used

$ git clone git@github.com:realm/SwiftLint.git; cd SwiftLint
$ swift build -c release -Xswiftc -target -Xswiftc x86_64-apple-macosx10.15
Fetching https://github.com/drmohundro/SWXMLHash.git
Fetching https://github.com/jpsim/SourceKitten.git
Fetching https://github.com/mattgallagher/CwlPreconditionTesting.git
Fetching https://github.com/scottrhoyt/SwiftyTextTable.git
Fetching https://github.com/Quick/Nimble.git
Fetching https://github.com/Quick/Quick.git
Fetching https://github.com/jpsim/Yams.git
Fetching https://github.com/mattgallagher/CwlCatchException.git
Fetching https://github.com/Carthage/Commandant.git
Completed resolution in 9.13s
Cloning https://github.com/mattgallagher/CwlCatchException.git
Resolving https://github.com/mattgallagher/CwlCatchException.git at 1.2.0
Cloning https://github.com/drmohundro/SWXMLHash.git
Resolving https://github.com/drmohundro/SWXMLHash.git at 5.0.1
Cloning https://github.com/jpsim/Yams.git
Resolving https://github.com/jpsim/Yams.git at 2.0.0
Cloning https://github.com/Carthage/Commandant.git
Resolving https://github.com/Carthage/Commandant.git at 0.17.0
Cloning https://github.com/Quick/Quick.git
Resolving https://github.com/Quick/Quick.git at 2.2.0
Cloning https://github.com/jpsim/SourceKitten.git
Resolving https://github.com/jpsim/SourceKitten.git at 0.29.0
Cloning https://github.com/scottrhoyt/SwiftyTextTable.git
Resolving https://github.com/scottrhoyt/SwiftyTextTable.git at 0.9.0
Cloning https://github.com/mattgallagher/CwlPreconditionTesting.git
Resolving https://github.com/mattgallagher/CwlPreconditionTesting.git at 1.2.0
Cloning https://github.com/Quick/Nimble.git
Resolving https://github.com/Quick/Nimble.git at 8.0.5
/Users/xxx/yyy/SwiftLint/Source/SwiftLintFramework/Extensions/Configuration+Cache.swift:13:9: warning: 'CC_MD5_Init' was deprecated in OS X 10.15: This function is cryptographically broken and should not be used in security contexts. Clients should migrate to SHA256 (or stronger).
        CC_MD5_Init(context)
...

Environment

• SwiftLint version 0.40.0
• Swift Environment (globally 5.2.4, but 5.0 at swiftlint dir)

$ swiftenv versions
  4.1
  4.1.2
  4.2
* 5.0 (set by /Users/xxx/yyy/SwiftLint/.swift-version)
  5.1
  5.2.4

[cfiken]

will be fixed by #3298