Restrict directories watched by guard

If we watch directories like .git or tmp, guard/listen can get very
slow when running tests generating new events, the adapter will have to
process all of them (and potentially search file system for new files).

Guardfile

@@ -1,3 +1,5 @@
+directories %w[features lib spec]
+
 guard :cucumber, cli: '--format pretty --quiet', all_on_start: false do
   watch(%r{\Afeatures/.+\.feature\z})
   watch(%r{\Afeatures/support/.+\.rb\z})                { 'features' }