Note that the next release is intended to be 5.0 to coincide with the
release of PostgreSQL 12; 4.5 is currently a placeholder in case we
need to push out a feature release before then.
repmgr requires that the same "major version" (e.g. 4.3) is present
on all nodes, otherwise - particularly in the case of repmgrd - it's
highly likely things won't work as expected.
Implements part of GitHub #515.
