Re-enable write barrier for concurrent marker.
Re-enable write barrier for concurrent marker.

kachick/rubinius,jsyeo/rubinius,jsyeo/rubinius,jsyeo/rubinius,kachick/rubinius,kachick/rubinius,jsyeo/rubinius,kachick/rubinius,jsyeo/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,jsyeo/rubinius,jsyeo/rubinius