Constrain directory_watcher to working version

If you run bundle update, the directory_watcher gem gets updated to version 1.5.1.  This is evidently incompatible with ruhoh because it throws an exception whenever a file changes, instead of reparsing.

This is on Windows with ruhoh 2.0.alpha.3.

This commit just locks the directory_watcher version to any 1.4.1+ release in the Gemfile and gemspec.