Rakefile: pacify RuboCop.
Rakefile: pacify RuboCop.

RuboCop prefers Dir.exist? over Dir.exists?.

Signed-off-by: brian m. carlson <738bdd359be778fee9f0fc4e2934ad72f436ceda@crustytoothpaste.net>

bk2204/daniel-ruby,bk2204/daniel-ruby