Compatibility with latest versions of jekyll

[csware]

It would be really nice if this plugin could be updated to work with the latest version of jekyll!

I just testet, that it breaks with jekyll >= 3.2.

[csware]

It seems to boil down to change lib\jekyll\site.rb as follows: add @includes_load_paths << File.join(source, File.join('../', config["includes_dir"].to_s)) at the end of def configure_include_paths

[sumdog]

Awesome, thanks. I've been working on a set of automated tests for jekyll-multisite in a local branch so I could do multi-version support (via multiple docker containers). I've been putting it off for a while, but I'm going to put it back on my priority list and try to get it checked in and setup in CI. I'll try to get your pull request reviewed in by this weekend I hope.