Create unity523.rb (#2434)

Added Version 5.2.3f for Unity.

Casks/unity523.rb

@@ -0,0 +1,14 @@
+cask 'unity523' do
+  version '5.2.3f1'
+  sha256 'a2cf141409078a8e1934a0dede357578580e318c21275696b57346acbcbd284c'
+
+  url "http://download.unity3d.com/download_unity/f3d16a1fa2dd/MacEditorInstaller/Unity-#{version}.pkg"
+  name 'Unity Editor'
+  homepage 'https://unity3d.com/unity/'
+  license :commercial
+
+  pkg "Unity-#{version}.pkg"
+
+  uninstall quit:    'com.unity3d.UnityEditor5.x',
+            pkgutil: 'com.unity3d.UnityEditor5.x'
+end