More concise Rubyish code.

Hash key checking is not necessary in this case.  If a key doesn't exist in a Hash, Ruby returns nil by default and nil is falsy. Hash key checking would only be necessary if the Hash had been set up with a default value.

05_hash_tables/ruby/02_check_voter.rb

@@ -3,7 +3,7 @@
 @voted = {}
 
 def check_voter(name)
-  if @voted.key?(name) && @voted[name]
+  if @voted[name]
     puts "kick them out!"
   else
     @voted[name] = true