spec/filters/keys_whitelist: fix sporadic segfault
spec/filters/keys_whitelist: fix sporadic segfault

With help of #324 I established that MRI 2.3 & 2.4 choke on the recursive hash
example on Circle CI. Example build:
https://circleci.com/workflow-run/c112358c-e7bf-4789-9eb2-4891ea84da68

It seems like a bug in the interpreter, which was fixed in 2.5, so we can ignore
the example on 2.3 & 2.4 (although it passes locally for me on 2.4 each time).

airbrake/airbrake-ruby,airbrake/airbrake-ruby