Don't try to load config from home directory if HOME isn't set

It's possible the HOME environment variable may not be set. jbundler shouldn't crash if it isn't.
Alternatively:

    homefile = File.join(ENV['HOME'], RC_FILE) if ENV.has_key? 'HOME'
    home_config = YAML.load_file(homefile) if (not homefile.nil?) and File.exists?(homefile)