no handling for change-deleted hook

[jf-ct]

https://gerrit.googlesource.com/plugins/hooks/+/refs/heads/stable-2.15/src/main/resources/Documentation/hooks.md#change_deleted

Traceback (most recent call last):
  File "...", line ..., in <module>
    gerrit_parser = gerrit_hooks.build_parser_for(args.gerrit_event)
  File "/usr/local/lib/python3.5/dist-packages/gerrit_hooks/hook.py", line 37, in build_parser_for
    cli_flags = HookFlagDefinitions()[hook_type]
  File "/usr/local/lib/python3.5/dist-packages/gerrit_hooks/containers.py", line 172, in __getitem__
    flag_string = self.__mapping__[item]
KeyError: 'change-deleted'

[deepbrook]

Should be fixed in the latest release!