Swift4finalmergecandidate minor fix

[isimple4]

Checklist

• [ ] New Extension
• [x] New Test
• [x] Changed more than one extension, but all changes are related
• [x] Trivial change (doesn't require changelog)

This PR mainly reverts some improper changes back to original, and fixes couple of warnings based on #476 .

Some changes explained as below.

• SWIFT_SWIFT3_OBJC_INFERENCE is set to Default as suggested here
• The Hashable and Equatable thing seems like an issue with XCode 9.2 compiler, I've revert them back and update to 9.3 for CI.
• #if targetEnvironment(simulator) is suggested to replace #if (arch(i386) || arch(x86_64)) && os(iOS) by compiler. Ref
• Block based NotificationCenter.​addObserver​(forName:​object:​queue:​using:) is warned by lint requiring an object to store the returned result, so that it can be removed later. I simply store it to an empty object, but here is a better solution.

[EZSwiftExtensionsBot]

	1 Error
:no_entry_sign:	Making non-trivial change requires changelog entry! Please, set trivial change or add entry to changelog.

	3 Messages
:book:	Executed 202 tests, with 0 failures (0 unexpected) in 4.858 (5.074) seconds
:book:	Executed 187 tests, with 0 failures (0 unexpected) in 4.518 (4.728) seconds
:book:	Executed 124 tests, with 0 failures (0 unexpected) in 4.054 (4.159) seconds

Generated by :no_entry_sign: Danger

[isimple4]

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