'Reads the compiled revision from the revision file.
Returns:
the revision of this vocabulary (i.e. the string
inside the revision file), or None if is_compiled
if False'

@property
def compiled_revision(self):

    if (not self.is_compiled):
        return None
    with open(self.revision_file, 'r') as f:
        revision = f.read().strip()
    self._logger.debug("compiled_revision    is    '%s'", revision)
    return revision
