swift version to 4.0

Author: Ryan Nystrom

MessageViewController.podspec

@@ -8,4 +8,5 @@ Pod::Spec.new do |spec|
   spec.source       = { :git => 'https://github.com/GitHawkApp/MessageViewController.git', :tag => '#{s.version}' }
   spec.source_files = 'MessageViewController/*.swift'
   spec.platform     = :ios, '10.0'
+  spec.swift_version = '4.0'
 end