Convert words to string before downcase to avoid type conversion errors.

I was receiving the following error on initialisation of the gem:

    ruby-1.9.2-p320/gems/zxcvbn-ruby-0.0.2/lib/zxcvbn/dictionary_ranker.rb:17:in `block in rank_dictionary': undefined method 'downcase' for 98765.0:Float (NoMethodError)

It seems that there was some automatic conversion of some dictionary words into `Float` type. This was fixed by explicitly converting each word to a string before executing `downcase`.