Use `plugins` instead of `plugins_dir` config
Use `plugins` instead of `plugins_dir` config

As mentioned in jekyll/jekyll#6195, since Jekyll 3.5.0 `plugin_dir` takes a string path to a plugins direcory, defaulting to `./_plugins`, and `plugins` takes a YAML array of plugins for Jekyll to load. Previously, `plugins` was a deprecated alias for `plugins_dir`, taking a string path, and `gems` was the array of plugins to load.
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