Update podspec to specify explicitly supported swift versions: ['4.2', '5.0', '5.1']

Author: lucdion

PinLayout.podspec

@@ -8,15 +8,15 @@
 
 Pod::Spec.new do |spec|
   spec.name          = "PinLayout"
-  spec.version       = "1.8.11"
+  spec.version       = "1.8.12"
   spec.summary       = "Fast Swift Views layouting without auto layout. No magic, pure code, full control and blazing fast."
   spec.description   = "Fast Swift Views layouting without auto layout. No magic, pure code, full control and blazing fast. Concise syntax, intuitive, readable & chainable. [iOS/macOS/tvOS/CALayer]"
   spec.homepage      = "https://github.com/layoutBox/PinLayout"
   spec.license       = "MIT license"
   spec.author        = { "Luc Dion" => "[email protected]" }
   spec.source        = { :git => "https://github.com/layoutBox/PinLayout.git", :tag => "#{spec.version}" }
   spec.source_files  = "Sources/**/*.swift"
-  spec.swift_version = '4.2'
+  spec.swift_version = ['4.2', '5.0', '5.1']
 
   spec.ios.deployment_target  = '8.0'
   spec.ios.frameworks         = 'Foundation', 'CoreGraphics', 'UIKit'