Henceforth, we would end up with `@mtimes = false`, and this is bad news for `#add`:
/Users/travis/.casher/bin/casherin `block in add': undefined method `[]=' for false:FalseClass (NoMethodError) from /Users/travis/.casher/bin/casherin `map' from /Users/travis/.casher/bin/casherin `add' from /Users/travis/.casher/bin/casher:52:in `block in run' from /Users/travis/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/1.9.1/timeout.rb:69:in `timeout' from /Users/travis/.casher/bin/casher:52:in `run' from /Users/travis/.casher/bin/casher:231:in `<main>'
We avoid this problem by doing two things:
1. Create @mtime_file only when md5deep is not available 2. Set @mtimes only when @mtime_file exists and it is not empty