Fix #37: Lock Listen to less than 3.1.
Fix #37: Lock Listen to less than 3.1.

Jekyll supports 2.1+ but @e2 has made it so that Listen only supports
2.2 so this locks it so that we are on a version of Listen that we can
actually support since we still support all supporte Ruby versions.

jekyll/jekyll-watch,jekyll/jekyll-watch,jekyll/jekyll-watch