Log Message:
www/ruby-jekyll-feed: update to 0.15.0

Update ruby-jekyll-feed package to 0.15.0.

## 0.15.0 / 2020-07-10

### Minor Enhancements

  * Add support for drafts (#316)

## 0.14.0 / 2020-06-24

### Minor Enhancements

  * add support for categories (#153) (#233)
  * add support for tags (#264)
  * Make posts limit configurable (#314)
  * XML escape the title field of feed_meta (#306)

### Bug Fixes

  * Fix feed link when post title contains HTML (#305)

### Development Fixes

  * Use Dir to list source files (#309)
  * Require Ruby >=2.4.0 (#307)