(fix) replace flatMap (node 11+) with flatten util

[dummdidumm]

No description provided.

[dummdidumm]

.. or we just keep it as is but require at least node 11. @ehrencrona thoughts?

[Conduitry]

I would vote for this PR rather than doing a major version bump and requiring Node 11. (I'm not sure whether there's already been a release that contains this code, but if so, it would be irresponsible not to release another patch fix even if we decided to go forward with a major version that notes the newer Node requirement.) I'd also vote for running CI on the oldest version of Node we say we support, to catch issues like this earlier.

[benmccann]

I think it's reasonable to support Node 10, which is still supported until 2021-04-30.

[dummdidumm]

flatMap apparently came in through #78, so since March. I agree with you, I'll merge this in.

[ehrencrona]

Since it’s also only about a single function that’s easy to replace I agree.