Fix truffleruby CI

[cben]

truffleruby is not very far from working. bundle install works and most tests pass, but CI [almost?] always fails.

Real failure:

  1) Failure:
KubeclientTest#test_not_upgraded_to_new_api [/home/beni/kubeclient/test/test_kubeclient.rb:53]:
Expected "second argument must be an api version like 'v1'" to include "wrong number of arguments".

Flakiness:

• #545 makes it sometimes timeout (SIGTERM after 3m).