Fix old 'prove' from erroring out on -j which was not included in, for example, 5.8.8 on perlbrew

Some of the older perl's that perlbrew fetches come with an older version of App::Prove. The readme example shows using "5.8.4" in combination with prove -l -j$((SYSTEM_CORES + 1)) which will give unknown options -j unknown option 3 unknown option 3 on travis-ci and fail. This patch checks for the version of prove, and uses $(test-jobs) to also pass the -j, such that if the version is below the threshold $(test-jobs) will be blank (complete with no -j flag). You probably want to change this so the user can still configure the number of cores they want to use instead of this PR using  a SYSTEMS_CORE + 1 default