Update for Swift 5.1 and Xcode 11 (#168)

* Rebuild dependencies with 5.1

* Import FoundationNetworking if found

SlackKit.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name                    = "SlackKit"
-  s.version                 = "4.3.0"
+  s.version                 = "4.5.0"
   s.summary                 = "Write Slack apps in Swift"
   s.homepage                = "https://github.com/pvzig/SlackKit"
   s.license                 = "MIT"
@@ -8,7 +8,7 @@ Pod::Spec.new do |s|
   s.source                  = { :git => "https://github.com/pvzig/SlackKit.git", :tag => s.version.to_s }
   s.social_media_url        = "https://twitter.com/pvzig"
   s.platforms               = { :ios => '10.0', :osx => '10.11', :tvos => '10.0' }
-  s.swift_version           = '5.0'
+  s.swift_version           = '5.1'
   s.cocoapods_version       = '>= 1.4.0'
   s.default_subspec         = "SlackKit"