test-bot: don't run readall on <=10.8.

The Ruby 1.8 implementation seems to be really flaky, unfortunately.

Library/Homebrew/cmd/readall.rb

@@ -22,7 +22,7 @@ module Homebrew
             while rb = ruby_files.pop(true)
               nostdout { failed = true unless system "ruby", "-c", "-w", rb }
             end
-          rescue ThreadError, IOError # ignore empty queue error
+          rescue ThreadError # ignore empty queue error
           end
         end
       end

Library/Homebrew/cmd/test-bot.rb

@@ -505,7 +505,9 @@ module Homebrew
     def homebrew
       @category = __method__
       test "brew", "tests"
-      test "brew", "readall", "--syntax"
+      readall_args = []
+      readall_args << "--syntax" if MacOS.version >= :mavericks
+      test "brew", "readall", *readall_args
     end
 
     def cleanup_before