Fixed VM.backtrace.
Fixed VM.backtrace.

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