Merge branch 'v2.4' of https://github.com/plumed/plumed2 into v2.4

.travis.yml

CHANGES/v2.3.md

CHANGES/v2.4.md

VERSION

configure

configure.ac

patches/gromacs-2016.6.config

patches/gromacs-2016.6.diff/src/gromacs/CMakeLists.txt

.../gromacs-2016.6.diff/src/gromacs/CMakeLists.txt.preplumed

patches/gromacs-2016.6.diff/src/gromacs/mdlib/force.cpp

...gromacs-2016.6.diff/src/gromacs/mdlib/force.cpp.preplumed

patches/gromacs-2016.6.diff/src/gromacs/mdlib/minimize.cpp

...macs-2016.6.diff/src/gromacs/mdlib/minimize.cpp.preplumed

patches/gromacs-2016.6.diff/src/programs/mdrun/md.cpp

...s/gromacs-2016.6.diff/src/programs/mdrun/md.cpp.preplumed

patches/gromacs-2016.6.diff/src/programs/mdrun/mdrun.cpp

...romacs-2016.6.diff/src/programs/mdrun/mdrun.cpp.preplumed

patches/gromacs-2016.6.diff/src/programs/mdrun/repl_ex.cpp

...macs-2016.6.diff/src/programs/mdrun/repl_ex.cpp.preplumed

patches/gromacs-2016.6.diff/src/programs/mdrun/repl_ex.h