replaced `Defaults\ *requiretty` to `Defaults:vagrant \!requiretty`.

I failed `vagrant provision` in the following environments.
```
Droplet info:
region: San Francisco 1
os: CentOS 7.0 x64
```

Failed logs:
```
$ vagrant provision
==> default: Installing rsync to the VM...
The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

yum -y install rsync

Stdout from the command:



Stderr from the command:

sudo: sorry, you must have a tty to run sudo
```

"Defaults requiretty" in "/etc/sudoers" is cause. so I wrote patch.
but, I think this patch is too bad solution. X(
I want to know if there is a better solution.

My Vagrantfile:
```ruby
Vagrant.configure('2') do |config|
  config.vm.hostname = '...'

  config.vm.provider :digital_ocean do |provider, override|
    override.ssh.username         = "root"
    override.ssh.private_key_path = '~/.ssh/id_rsa'
    override.vm.box               = 'digital_ocean'
    override.vm.box_url           = "https://github.com/smdahlen/vagrant-digitalocean/raw/master/box/digital_ocean.box"


    provider.client_id    = '...'
    provider.api_key      = '...'
    provider.token        = '...'
  end
end
```

Will succeed if patched.
```
 $ vagrant provision
==> default: Installing rsync to the VM...
==> default: Rsyncing folder: /Users/hisaichi5518/project/demo/ => /vagrant...
```