Judging from [this discussion](https://github.com/jekyll/jekyll/issues/4105) this is a common issue with plugins because `/lib/jekyll/post.rb` no longer exists in 3.0.0 as it did in 2.5.3. Some example fixes for similar problems are at [https://gist.github.com/uraimo/aa1437323a5da43c86cf](https://gist.github.com/uraimo/aa1437323a5da43c86cf).