Fix swift < 4.2 compilation issues.

Author: alexdrone

RenderNeutrino.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = "RenderNeutrino"
-  s.version          = "5.5.1"
+  s.version          = "5.5.2"
   s.summary          = "Render is a declarative library for building efficient UIs on iOS inspired by React."
   s.description      = <<-DESC
   s.platform         = :ios

render/mods/stylesheet/src/Stylesheet.swift

@@ -3,8 +3,10 @@ import UIKit
 
 // MARK: - UIStylesheet
 
+#if swift(>=4.2)
 /// Shorthand to access the default global stylesheet.
 public let S = UIStylesheet().style
+#endif
 
 public struct UIStylesheet {
   /// Returns the stylesheet rule for the given path (e.g. Palette.black).