ENH: Use Slicer/ParameterSerializer and update to HEAD.

The upstream repository location is now github.com/Slicer/ParameterSerializer.

Also update the ExternalProject version to latest HEAD which contains
the latest fixes for Slicer.
Merge pull request #292 from thewtex/ParameterSerializerSlicer

ENH: Use Slicer/ParameterSerializer and update to HEAD.