Bump rexml #656

Job	Run time
omnibus compilations, trigger	3s
omnibus compilations, #4	32m 0s
omnibus compilations, #2	33m 10s
omnibus compilations, #3	33m 49s
omnibus compilations, #1	30m 3s
omnibus compilations, #9	27m 50s
omnibus compilations, #12	14m 36s
omnibus compilations, #8	28m 41s
omnibus compilations, #6	28m 29s
omnibus compilations, #10	26m 43s
omnibus compilations, #5	27m 38s
omnibus compilations, #11	23m 25s
omnibus compilations, #7	33m 6s
omnibus compilations, result	3s
	5h 39m 36s

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