gromacs: 2018.1 -> 2018.2 (#42504)

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/gromacs/versions.

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2/bin/GMXRC had a zero exit code or showed the expected version
- /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2/bin/GMXRC.bash passed the binary check.
- /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2/bin/GMXRC.zsh passed the binary check.
- Warning: no invocation of /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2/bin/GMXRC.csh had a zero exit code or showed the expected version
- Warning: no invocation of /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2/bin/demux.pl had a zero exit code or showed the expected version
- Warning: no invocation of /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2/bin/xplor2gmx.pl had a zero exit code or showed the expected version
- Warning: no invocation of /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2/bin/gmx-completion.bash had a zero exit code or showed the expected version
- Warning: no invocation of /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2/bin/gmx-completion-gmx.bash had a zero exit code or showed the expected version
- /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2/bin/gmx passed the binary check.
- 3 of 9 passed binary check by having a zero exit code.
- 0 of 9 passed binary check by having the new version present in output.
- found 2018.2 with grep in /nix/store/illh3rp28ipz35z0wipsfc18z4m03sah-gromacs-2018.2
- directory tree listing: https://gist.github.com/1679f0362dfcc7299a6d5b8d9e76c14e
- du listing: https://gist.github.com/2f848bb0fe181676c17b61a9d5a94953