Relax pinning of net-ssh to ~> 2.6.5 for compatibility with Chef 10.22.0

Chef 10.22.0 and test-kitchen require net-ssh ~> 2.6, so they won't work with Vagrant:

```$ kitchen test
/Users/jdunn/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/rubygems/specification.rb:1637:in `raise_if_conflicts': Unable to activate chef-10.22.0, because net-ssh-2.2.2 conflicts with net-ssh (~> 2.6) (Gem::LoadError)
```

I tested vagrant with net-ssh 2.6.5 and it works great, so I think it's safe to bump it.