test-bot: don't cleanup before on Travis.

As we may be using a Ruby 1.8 that this will delete.

Library/Homebrew/cmd/test-bot.rb

@@ -647,7 +647,7 @@ module Homebrew
       git "rebase", "--abort"
       git "reset", "--hard"
       git "checkout", "-f", "master"
-      git "clean", "-ffdx"
+      git "clean", "-ffdx" unless ENV["TRAVIS"]
       pr_locks = "#{HOMEBREW_REPOSITORY}/.git/refs/remotes/*/pr/*/*.lock"
       Dir.glob(pr_locks) { |lock| FileUtils.rm_rf lock }
     end