Changing the conditions on which hash functions for transformations and partial perms are defined.

Instead of comparing the version number, IsBound(IsPPerm2Rep) and
IsBound(IsTrans2Rep) are used for post-4.7 but pre-4.7.5 GAP. This
allows Orb to be loaded in the stable-4.6 branch of the GAP repo, as
request in pull request #1.