Update versioning for new scheme

Update versioning for new scheme

Make versioning use the new YYYY[.P] scheme that was decided quite a
while back.

Also, remove the literal "VERSION" prefix from the return value of
gmx_version() to give caller better control for deciding on the usage of
the version string.

Change-Id: I1ed171502a4a900c87988faabf48fafeef735c4f