[ruby/irb] Use Kernel.warn to print command alias warning

(https://github.com/ruby/irb/pull/601)

This aligns with other warnings in irb and properly channel the message
to stderr.

lib/irb/extend-command.rb

@@ -289,7 +289,7 @@ module IRB # :nodoc:
           alias_method to, from
         }
       else
-        Kernel.print "irb: warn: can't alias #{to} from #{from}.\n"
+        Kernel.warn "irb: warn: can't alias #{to} from #{from}.\n"
       end
     end