Mark Swift 5 as supported (#2730)

* Mark Swift 5 as supported

* Only set a single Swift version on podspec

Multiple swift versions requires CocoaPods 1.7.0, which is still
in beta.

* Modernize ruby hash syntax

* Update SourceKitten to 0.23.1

SwiftLint.podspec

@@ -3,9 +3,9 @@ Pod::Spec.new do |s|
   s.version        = `make get_version`
   s.summary        = 'A tool to enforce Swift style and conventions.'
   s.homepage       = 'https://github.com/realm/SwiftLint'
-  s.license        = { :type => 'MIT', :file => 'LICENSE' }
+  s.license        = { type: 'MIT', file: 'LICENSE' }
   s.author         = { 'JP Simard' => 'jp@jpsim.com' }
-  s.source         = { :http => "#{s.homepage}/releases/download/#{s.version}/portable_swiftlint.zip" }
+  s.source         = { http: "#{s.homepage}/releases/download/#{s.version}/portable_swiftlint.zip" }
   s.preserve_paths = '*'
   s.exclude_files  = '**/file.zip'
 end