Gemfile: update branch-name of 'vagrant-spec' (#1163)
Gemfile: update branch-name of 'vagrant-spec' (#1163)

fixes `bundle install` step from the README, currently broken by https://github.com/hashicorp/vagrant-spec/issues/41

```
~/src/vagrant-libvirt $ bundle install
....
Fetching git://github.com/hashicorp/vagrant-spec.git
fatal: Needed a single revision
Revision master does not exist in the repository git://github.com/hashicorp/vagrant-spec.git. Maybe you misspelled it?
```


Works fine now on my workstation with the new name:
```
Using vagrant-spec 0.0.1 from git://github.com/hashicorp/vagrant-spec.git (at main@b6b56c8)
Bundle complete! 10 Gemfile dependencies, 73 gems now installed.
```
pradels/vagrant-libvirt,pradels/vagrant-libvirt,electrofelix/vagrant-libvirt,electrofelix/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,electrofelix/vagrant-libvirt