Remove .swift-version, use Podspec swift_versions

[bobgilmore]

Whenever we update the pod and publish to Cocoapods,
pod spec lint
warns us that

Usage of the .swift_version file has been deprecated!...

Let's clean it up and replace it with swift_versions in the podspec
per lint's recommendation.

See issue #169 in the repo.

[ipbuildserver]

	5 Warnings
⚠️	ResultTests.swift#L15 - SwiftWisdomTests/CommonTypes/ResultTests.swift#L15: ‘IPResult’ is deprecated: use Swift.Result instead private final let successfulResult = IPResult<Int>.success(1)
⚠️	ResultTests.swift#L16 - SwiftWisdomTests/CommonTypes/ResultTests.swift#L16: ‘IPResult’ is deprecated: use Swift.Result instead private final let failureResult = IPResult<Any>.failure(ResultError.error)
⚠️	UnsignedInteger+ExtensionsTests.swift#L16 - SwiftWisdomTests/StandardLibrary/Numbers/UnsignedInteger+ExtensionsTests.swift#L16: ‘random(inRange:)’ is deprecated: Use Int.random(in:) instead XCTAssert(positives.contains(random(inRange: positives)))
⚠️	UnsignedInteger+ExtensionsTests.swift#L18 - SwiftWisdomTests/StandardLibrary/Numbers/UnsignedInteger+ExtensionsTests.swift#L18: ‘random(inRange:)’ is deprecated: Use Int.random(in:) instead XCTAssert(negatives.contains(random(inRange: negatives)))
⚠️	UnsignedInteger+ExtensionsTests.swift#L20 - SwiftWisdomTests/StandardLibrary/Numbers/UnsignedInteger+ExtensionsTests.swift#L20: ‘random(inRange:)’ is deprecated: Use Int.random(in:) instead XCTAssert(mixed.contains(random(inRange: mixed)))

	1 Message
📖	Executed 174 tests, with 0 failures (0 unexpected) in 0.429 (0.488) seconds

Generated by 🚫 Danger

[bobgilmore]

I didn't update the version in the "main" Podspec, because PR 170 handles that for me. I'll merge this one, then tag this one, merge, and push to trunk.