change example code to do mrb_close()

I was hoping this would cause valgrind to complain less, but there is still
a lot of memory leaked (does mrb_close() actually free all of the managed
blocks?)  Anyway this helps somewhat and is good practice

test/driver.c

@@ -55,6 +55,7 @@ main(void)
   else {
     /* no */
   }
+  mrb_close(mrb);
 
   return 0;
 }