Update heapdump.cc

[rwalle61]

Leftover work from https://github.com/RuntimeTools/appmetrics/pull/576

Re. https://github.com/RuntimeTools/appmetrics/pull/576#discussion_r287450014, rather than updating heapdump.cc ourselves, we should grap the latest one maintained at https://github.com/bnoordhuis/node-heapdump/blob/master/src/heapdump.cc