message: Document change for middleman.heartbeat-interval
diff: @@ -225,6 +225,8 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  - Using unquoted strings in command-line arguments inside lists now works as
  expected. For example, `--foo=abc,def` is now equivalent to
  `--foo=["abc", "def"]`.
  +- Fixed a type mismatch in the parameter `middleman.heartbeat-interval` (#1095).
  + CAF consistently uses `timespan` for this parameter now.
  ## [0.17.4] - 2019-02-08
  