disable sandbox for 10.5

Although `sandbox-exec` is available in 10.5, it has very limit
support on the DSL for sandbox profile file(.sb file). Therefore,
sandbox for Homebrew on 10.5 is broken.

Closes #735.

Signed-off-by: Xu Cheng <git@xuc.me>

Library/Homebrew/sandbox.rb

@@ -8,7 +8,7 @@ class Sandbox
   ].freeze
 
   def self.available?
-    OS.mac? && File.executable?(SANDBOX_EXEC)
+    OS.mac? && OS::Mac.version >= "10.6" && File.executable?(SANDBOX_EXEC)
   end
 
   def self.formula?(formula)