I think this happens because you have vim 2:7.4.488-7 (from jessie)
installed, which uses libruby2.1. vim in testing/unstable uses libruby2.2, so upgrading to this version would get things working.
It would, but the packaging system should ensure that whatever version of vim is installed, the program will work or won't be installable.
In fact, I'll probably just remove the version requirements on those
build-dependencies entirely--they just get in the way when backporting
the package to run on jessie.
Are you sure? Isn't this a linking problem?
To me the plugin is linked to ruby 2.2 and vim linked to ruby 2.1
It sort of needs a dpkg-shlibdeps type of thing going on, but with vim not ldd.
- Craig