pythonPackages.distributed: add missing dependencies

Added `mpi4py` and `bokeh` to get the executables working.

Also simplified the `lib.optional` expression as Python 3.2 and 3.4
aren't supported anymore. Rather than referencing unsupported Python 3.x
versions it's far more obvious now to only use `futures` and
`singledispatch` if Python2 is used.