This is the first stable version of gerbil, named 0.12-RELEASE
so that builtins.compareVersions should consider it newer than the
packages of the 0.12-DEV-xxx series.
Next stable package version will be gerbil-0.13 without -RELEASE.
Next unstable package version will be in its own distinct package
gerbil-unstable-2018-02-28 or some such, as per nixpkgs guidelines.