Use a rb_ary_entry and not RARRAY_PTR()

For MRI, this doesn't make a difference, but on Rubinius this prevents
having to setup a backing store for copying over the internal array
pointers. This was the only case where RARRAY_PTR was used, other places
already use rb_ary_entry, so it should be a good change for consistency
as well.
Merge pull request #877 from rubinius/master

Use rb_ary_entry instead of RARRAY_PTR