Samtools Idxstats version check

Only evaluate r### format samtools version numbers.  Newer versions contain Idxstats sub-command.

There might be better ways, but hoping this is a quick fix for this specific problem.
The only place this is used in a pipeline is in Genome::Model::Tools::DetectVariants2::Breakdancer _get_chr_list.  Even worse, it doesn't define a version of Samtools at all.