test-bot: don't try and bottle unstable formulae.

e.g. devel-only or head-only formulae.

Library/Homebrew/cmd/test-bot.rb

@@ -447,7 +447,7 @@ module Homebrew
       audit_args << "--strict" if @added_formulae.include? formula_name
       test "brew", "audit", *audit_args
       if install_passed
-        unless ARGV.include? '--no-bottle'
+        if formula.stable? && !ARGV.include?('--no-bottle')
           bottle_args = ["--rb", formula_name]
           if @tap
             tap_user, tap_repo = @tap.split "/"