Added osx, tvOS and watchOS to podspec file (#35)

Author: Andrew-Lees11

SwiftJWT.podspec

@@ -6,7 +6,10 @@ Pod::Spec.new do |s|
   s.license      = { :type => "Apache License, Version 2.0" }
   s.authors      = 'IBM'
   s.module_name  = 'SwiftJWT'
+  s.osx.deployment_target = "10.12"
   s.ios.deployment_target = "10.3"
+  s.tvos.deployment_target = "10.3"
+  s.watchos.deployment_target = "3.3"
   s.source       = { :git => "https://github.com/IBM-Swift/Swift-JWT.git", :tag => s.version }
   s.source_files  = "Sources/**/*.swift"
   s.dependency 'BlueRSA', '~> 1.0'