Allowed Swift 4.0, 4.2 and 5.0 in podspec (requires Cocoapods 1.7)

Author: rhodgkins

HTTPStatusCodes.podspec

@@ -8,7 +8,7 @@ Pod::Spec.new do |s|
   s.authors = 'Rich Hodgkins'
   s.source = { :git => 'https://github.com/rhodgkins/SwiftHTTPStatusCodes.git', :tag => s.version }
 
-  s.swift_versions = '5.0'
+  s.swift_versions = ['4.0', '4.2', '5.0']
 
   s.ios.deployment_target = '8.0'
   s.osx.deployment_target = '10.9'