*Always* ignore .jekyll-metadata, even if it doesn't exist.

Addresses [this comment](https://github.com/jekyll/jekyll-watch/pull/15#issuecomment-65860347):

> Edge case bug I just realized: the first time the site is run with jekyll [blah] --watch, it'll continuously regenerate because it doesn't add .jekyll-metadata unless it's there.

Follow-up to #15.