(RE-3358) Move versionbump from Pkg::Tar to Pkg:Util:File.

Prior to this commit, versionbump was occurring in Pkg::Tar, but not
Pkg::Util::File. This resulted in missing versionbumps for some artifacts,
such as gems. This commit moves the versionbump to Pkg:Util:File, where
it will versionbump the appropriate artifacts.