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.
If you prefer, I think  you can combine this on one line as 

`die $self->error_message('samtools version: '. $self->use_version .' does not have idxstats option. Please provide a more recent verison of samtools.');` 

As far as I'm concerned though, its personal preference.