Future-proof this conditional

Library/Homebrew/extend/ENV/super.rb

@@ -86,7 +86,7 @@ module Superenv
 
     # On 10.9, the tools in /usr/bin proxy to the active developer directory.
     # This means we can use them for any combination of CLT and Xcode.
-    self["HOMEBREW_PREFER_CLT_PROXIES"] = "1" if MacOS.version == "10.9"
+    self["HOMEBREW_PREFER_CLT_PROXIES"] = "1" if MacOS.version >= "10.9"
 
     # The HOMEBREW_CCCFG ENV variable is used by the ENV/cc tool to control
     # compiler flag stripping. It consists of a string of characters which act