Fix m_cost check

The invalid `m_cost` exception was checking against `t_cost`.

Author: ndemonner

lib/argon2.rb

@@ -11,7 +11,7 @@ def initialize(options = {})
       @t_cost = options[:t_cost] || 2
       raise ArgonHashFail, "Invalid t_cost" if @t_cost < 1 || @t_cost > 10
       @m_cost = options[:m_cost] || 16
-      raise ArgonHashFail, "Invalid m_cost" if @t_cost < 1 || @t_cost > 31
+      raise ArgonHashFail, "Invalid m_cost" if @m_cost < 1 || @m_cost > 31
       @salt = options[:salt_do_not_supply] || Engine.saltgen
       @secret = options[:secret]
     end