Require Swift 5.0 to build

[jpsim]

• Update READMEs
• Remove Swift 4.x compatibility shims
• Update CI
  • Stop testing with Swift 4.x & Xcode 10.0/10.1
  • Use official Swift docker image instead of norionomura's
  • Use Xcode 10.3 as latest stable version

[SwiftLintBot]

	12 Messages
📖	Linting Aerial with this PR took 3.11s vs 3.42s on master (9% faster)
📖	Linting Alamofire with this PR took 6.88s vs 7.06s on master (2% faster)
📖	Linting Firefox with this PR took 18.2s vs 19.34s on master (5% faster)
📖	Linting Kickstarter with this PR took 33.29s vs 29.5s on master (12% slower)
📖	Linting Moya with this PR took 3.0s vs 2.93s on master (2% slower)
📖	Linting Nimble with this PR took 2.57s vs 2.42s on master (6% slower)
📖	Linting Quick with this PR took 0.91s vs 0.91s on master (0% slower)
📖	Linting Realm with this PR took 4.61s vs 4.2s on master (9% slower)
📖	Linting SourceKitten with this PR took 1.81s vs 1.72s on master (5% slower)
📖	Linting Sourcery with this PR took 5.34s vs 5.15s on master (3% slower)
📖	Linting Swift with this PR took 46.81s vs 39.92s on master (17% slower)
📖	Linting WordPress with this PR took 44.54s vs 40.45s on master (10% slower)

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