upgrade dependencies, most notably: jcmturner/gokrb5(v4->v5)

[jcmturner]

Update to v5 of gokrb5.

I have not been able to test the plugin as I don't have an environment to test in.

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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[jcmturner]

Hi, this seems to work for upgrading the version of gokrb5 from v4 to v5. However I can't agree to the CLA as it implies that I am giving up all rights to the code that is checked in to this repo. This may be interpreted to include that which is in the vendor directory.

[pault28]

@jcmturner I will give this a test as we actively use this plugin. See now closed jcmturner/gokrb5#156. Also this is related to #9

@ah- Any chance of reviewing this?

[ah-]

Wonderful, thanks for this! Changes look good, if @pault28 s testing works out I'll merge.

[pault28]

@ah- this works perfectly fine for windows and linux clients. +1 to merge.

[ah-]

All there now, thanks for testing @pault28!