Pin chef-metal-vagrant and cheffish gems

If you run `rake bundle` with the current Gemfile you'll install incompatible `cheffish` and `chef-metal-vagrant` gems that break the machine resource.

```
NoMethodError
-------------
undefined method `chef_metal' for #<Chef::RunContext:0x007fdb14acba60>


Cookbook Trace:
---------------
  /Users/ryan/.chef/local-mode-cache/cache/cookbooks/ec-harness/recipes/vagrant.rb:8:in `from_file'
  /Users/ryan/.chef/local-mode-cache/cache/cookbooks/ec-harness/recipes/private_chef_ha.rb:3:in `from_file'
```