(Maint) Don't abort if gembuilddir doesn't exist

Previously, running `rake package:gem` from facter would fail with the
error:

    $ rake package:gem
    rm -rf pkg
    mkdir -p pkg
    WARNING:  no rubyforge_project specified
    mv facter-1.7.4.38.gem pkg/facter-1.7.4.38.gem
    rm -r pkg/facter-1.7.4.38
    rake aborted!
    No such file or directory - pkg/facter-1.7.4.38

This changes the gem task to rm -rf pkg/facter-1.7.4.38 instead. I don't
know why the same error doesn't occur when creating a puppet gem.