Make ps execution module available for psutil>=0.3.0

After looking at some of the older source code versions, it seems that
the majority (if not all) of the features present in the ps execution
module became available between psutil release 0.2.0 and 0.3.0. This
commit removes the python version check in the __virtual__ function, and
replaces it with a check of the psutil version. Only psutil versions
greater than or equal to 0.3.0 will result in the ps execution module
being available.

Fixes #7432.