The DBM module of Ruby  used to be built against db4 and later
against libdb packages. However, for some while already, it is
compatible even with gdbm. I'm thinking about dropping the
BuildRequires: libdb-devel and keep just the BuildRequires: gdbm-devel
(this is used for GDBM module anyway).
* One less dependency
* I've heard from libdb packager, that libdb should be dropped from
Fedora sooner or later, so it would not cause any future issues.
* Although the DBM Ruby interface does not change, the data files will
change due to this, i.e. if you are using DBM and you'll update from F21
to F22, this will be incompatible change.
I am not aware of any SW, which depends on DBM, so I am in favor for
this switch for Ruby 2.2 and F22. Furthermore, the above mentioned
transition from db4 to libdb went unnoticed. But please, let me know if
you have any concern.