Update to Psych 4.0.x

[marksiemers]

Psych.safe_load changed it's method signature and requires keyword args for 'permitted_classes'

Update the calls to Psych.safe_load and add it as an explicit gem dependency with a pessimistic lock on the version.

[marksiemers]

@pluff or @jaimerson - Any chance this could be reviewed?

[mculp]

👀