Fix string representation for actor.

[aequitas]

Fixes:

    def __str__(self):  # pragma: no cover
>       return "Actor(%(actor_name))" % vars(self)
E       ValueError: unsupported format character ')' (0x29) at index 19

[Bogdanp]

Thank you! I've merged your changes with rebase in 36f64e6 and 151403f and will release version 0.19.1 in a few minutes.