Remove OrderedDict from PriorDicts

From python3.6+, all dicts are ordered by design. On the otherhand, the
collections OrderedDict has odd behaviour when pickled (see comment
below). This changes to just using dicts. For users of python3.5 and
earlier, this will only change the order of their output posterior. On
the other hand, it will make pickling priors much more straight forward
going forward.