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Remove deprecated .swift-version file

Bump version & update changelog

Adds support for Swift 4.2 and Xcode 10.

- Upgrade Result dependency to 4.0.
- [FIX] Fixed a typo and incorrect reference in the `andThen` test ([https://github.com/Thomvis/BrightFutures/pull/197](#197), thanks [https://github.com/robertoaceves](robertoaceves)!)

Tag for 6.0.1

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