Don't rename the config file if there is nothing to migrate from

I am a new user of beets (since v1.1.0). I was playing around with beets and found that if I had already created a config.yaml file and issued the 'beet migrate' command I would end up with no config at all. This happens because it renames the config.yaml file before checking if the old style config exists. This simple patch should fix this problem.