Fix ssh boolean values in Vagrantfile template

[zuazo]

This PR fixes boolean values inside the ssh: configuration key. For example:

---
driver:
  ssh:
    insert_key: false

This will create a Vagrantfile like the following:

Vagrant.configure("2") do |c|
  c.ssh.insert_key = "false"
end

Instead of:

Vagrant.configure("2") do |c|
  c.ssh.insert_key = false
end

This PR fixes that little problem.

Bug mentioned in #163.