make gem dependencies reasonably optimistic again (fixes #253)

This reverts the vagrant-libvirt.gemspec changes in commit
19cd8be193fdeb0a8e1b004705ef3d4465c02812.
remove pry debug
Merge pull request #255 from aspiers/remove-debug

remove pry debug
Merge pull request #254 from aspiers/loosen-deps

make gem dependencies reasonably optimistic again (fixes #253)