update default xcode recipe to handle all three platforms

recipes/xcode.rb

@@ -1,3 +1,21 @@
-xcode node['macos']['xcode']['version'] do
-  ios_simulators node['macos']['xcode']['simulator']['major_version']
+if node['platform_version'].match? Regexp.union '10.13'
+  execute 'Disable Gatekeeper' do
+    command ['spctl', '--master-disable']
+  end
+
+  xcode node['macos']['xcode']['version']
+
+elsif node['platform_version'].match? Regexp.union '10.12'
+  execute 'Disable Gatekeeper' do
+    command ['spctl', '--master-disable']
+  end
+
+  xcode '9.2' do
+    ios_simulators %w(11 10)
+  end
+
+elsif node['platform_version'].match? Regexp.union '10.11'
+  xcode '8.2.1' do
+    ios_simulators %w(10 9)
+  end
 end