Fix UnknownMessageError import path

[kdmccormick]

This PR fixes an issue where I was while running into while trying to run the latest pylint (2.4.1) with the latest pylint-django (2.0.11):

Traceback (most recent call last):
  File "/home/kyle/worktracker/venv/lib/python3.7/site-packages/pylint_plugin_utils/__init__.py", line 3, in <module>
    from pylint.utils import UnknownMessage
ImportError: cannot import name 'UnknownMessage' from 'pylint.utils' (/home/kyle/worktracker/venv/lib/python3.7/site-packages/pylint/utils/__init__.py)

I did some digging, and it looks like UnknownMessageError has lived in pylint.exceptions since Pylint v1.7: https://github.com/PyCQA/pylint/blob/033201f7216512757dcda1f09ea6d4f7ba9fc2e3/pylint/utils.py#L36. However, this library continued to work because Python allows you to import names that have been imported into a module. Around Pylint 2.4, the utils module was refactored in such a way that it didn't expose `UnknownMessageError , which broke this library.

This PR fixes the import path, and bumps the version from 0.5 to 0.6.

[kdmccormick]

Closed because this duplicates #16