Fixes for 0.7.9.1 release (=> 0.7.9.2)

Andy;
Thanks for all your work on Platypus. I was testing out the new awesome stdout functionality in 0.7.9.2 and ran into a couple of issues, fixed in this pull request:

- Avoid calling mergeVCFfiles when outputing variants to standard out
- Clean up recent isValidVcfLine addition to be a non-class function and use correct local name (`line`)

As another small suggestion, you might want to use GitHub tags for Platypus releases to avoid having a big development repository with all of the tarballs. As an example, I pushed 0.7.9.2 to my fork so we could use it for bcbio integration:

https://github.com/chapmanb/Platypus/releases

Thanks again for the great tool.