Exclude vendor directory from Jekyll

When using this app with Jekyll 1.2.1, `jekyll serve --watch` complains that the site could not be built because `0000-00-00-welcome-to-jekyll.markdown.erb` does not have a valid date, and therefore is not syntactically valid. This fix adds the `vendor` directory to Jekyll's exclude list in `_config.yml`. Jekyll serving then works fine.

I'm opening this for comments.