Enable building documentation with Sphinx < 2.0.0

Enable building documentation with Sphinx < 2.0.0

The ReferenceRole class is only available in Sphinx >= 2.0.0, which
makes building BIND 9 documentation impossible with older Sphinx
versions:

    Running Sphinx v1.7.6

    Configuration error:
    There is a programable error in your configuration file:

    Traceback (most recent call last):
      File "/usr/lib/python3.6/site-packages/sphinx/config.py", line 161, in __init__
        execfile_(filename, config)
      File "/usr/lib/python3.6/site-packages/sphinx/util/pycompat.py", line 150, in execfile_
        exec_(code, _globals)
      File "conf.py", line 21, in <module>
        from sphinx.util.docutils import ReferenceRole
    ImportError: cannot import name 'ReferenceRole'

Work around the problem by defining a stub version of the ReferenceRole
class if the latter cannot be imported.  This allows documentation
(without GitLab hyperlinks in release notes) to be built with older
Sphinx versions.