Use forked gem for kubernetes client

What does this MR do and why?

The kubeclient gem has some dependencies that we'd like to see updated. On GitHub where the gem's git repository resides, the master branch has the dependencies updated, but the change has not yet been published to RubyGems. Ordinarily, we don't fetch gems from git repositories, but in this case I have forked the relevant repository myself and would like for us to consider using this version until the kubeclient gem is able to publish a release to RubyGems, at which point we can switch back.

This MR points the gem to the forked repository's commit SHA that gets the dependency updates.

Once a release is eventually published, we can move back to using the gem from RubyGems.

Reviewer note

This will include some other changes to the gem since it's not a back-port to the version we're already on.