1.4.0

Implemented enhancements:

• Remove flatMap because it is now deprecated in Swift 4.1 #71 (sebleclerc)
• Swift 4.0 Support #64 (nlutsenko)
• Adding a new executor which takes an escaping closure #58 (sebleclerc)

Merged pull requests:

• Changed casing of Executor enum value in README. #56 (sjorsvb)

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