Reading Time bug

[mmistakes]

If words_per_minute is not set in _config.yml and a post has read_time in the Front Matter build will fail with the following error:

Configuration file: /Users/mac/gits/jekyll/minimal-mistakes/_config.yml
            Source: /Users/mac/gits/jekyll/minimal-mistakes
       Destination: /Users/mac/gits/jekyll/minimal-mistakes/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
  Liquid Exception: divided by 0 in _includes/archive-single.html, included in page6/index.html
jekyll 3.0.3 | Error:  divided by 0

[mmistakes]

Ref https://github.com/mmistakes/jekyll-theme-unit-test/issues/1