Replace event-stream with something supported and secure

[jeremywadsack]

As mentioned in #33 and addressed in #34 dominictarr/event-stream had a supply chain attack that infected a version of release. The solution was to pin to an older version. The author subsequently released a final version 4.0.1 which does not have the issue, however they stopped supporting the module in 2016 and there are suggestions of other packages here: dominictarr/event-stream#102 (comment). In 2018 the author archived the repo.

It would be great if ps-tree would be updated to depend on a library that is currently maintained for security.