Add minimum firmware flag to Python SPK

Using the DSM5 toolchains, the Python SPK does not work with older DSM versions. Added a minimum firmware version (DSM5 beta), to indicate this during installation of the SPK.

Other packages might be affected, although it'll need to be tested (haven't started that). 

For people who want to stay on DSM4.x for whatever reason, we could offer an unmaintained DSM4 version in the package repo or on the site as direct download.