Remove references to www.prisonersdilemma.com

[marcharper]

The domain apparently has new ownership and no longer links to a list of strategies. See https://axelrod.readthedocs.io/en/fix-documentation/reference/overview_of_strategies.html#hard-tft for an example of where we link to this website.

[marcharper]

The original source for some of these strategies appears to be the PRISON library, which was formerly available at https://www.lifl.fr/IPD

See also the paper "Adaptive Behaviour in the Classical Iterated Prisoner’s Dilemma".