blocks upgrade: MDL-20820 another blocks upgrade problem with sticky blocks

Even though the visible column is going to be dropped,
we need to set it here or the query at line 2087 will fail.

Huge thanks to Oleg Sychev who did all the debugging and testing for this fix.

lib/db/upgrade.php

@@ -2043,6 +2043,7 @@ WHERE gradeitemid IS NOT NULL AND grademax IS NOT NULL");
             $newblock->defaultregion = $newregions[$stickyblock->position];
             $newblock->defaultweight = $stickyblock->weight;
             $newblock->configdata = $stickyblock->configdata;
+            $newblock->visible = 1;
             $DB->insert_record('block_instances', $newblock);
         }