Fix podspec versions (#747)

Co-authored-by: Robert Payne <robertjpayne@Roberts-MacBook-Pro.local>

SnapKit.podspec

@@ -14,5 +14,5 @@ Pod::Spec.new do |s|
 
   s.source_files = 'Sources/*.swift'
 
-  s.swift_version = '5.0'
+  s.swift_versions = ['5.0']
 end