gromacs: add v2019.6 (#15267)
gromacs: add v2019.6 (#15267)

* gromacs: add v2019.6

* Update var/spack/repos/builtin/packages/gromacs/package.py

Co-Authored-By: Adam J. Stewart <023e75a89b1f5ca1e59909f8756c185885836ad9@gmail.com>

Co-authored-by: Adam J. Stewart <023e75a89b1f5ca1e59909f8756c185885836ad9@gmail.com>

iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack