Fix exception error message in Dependency#new

which was always printing 'nil' for the invalid dependency type as it was attempting to display @type before it had been initialized.  Use the type parameter instead.

lib/rubygems/dependency.rb

@@ -54,7 +54,7 @@ def initialize name, *requirements
 
     unless TYPES.include? type
       raise ArgumentError, "Valid types are #{TYPES.inspect}, "
-        + "not #{@type.inspect}"
+        + "not #{type.inspect}"
     end
 
     @name        = name