Use Xcode 11.3.1 for xcframework building

fastlane/Fastfile

@@ -106,7 +106,10 @@ platform :ios do
   lane :xcframework do |options|
     target_version = "Kingfisher-#{options[:version]}"
     supporting_root = "../build/#{target_version}/Supporting Files"
-    xcversion(version: "~> 11.0")
+    
+    # Xcode 11.4 causes a crash in the final xcframework
+    # https://github.com/onevcat/Kingfisher/issues/1436
+    xcversion(version: "11.3.1")
     FileUtils.rm_rf '../build'
     
     frameworks = []