Change regex to match more vagrant machine names

serverspec-init recognizes `/[a-z_-]+/` as vagrant machine name.
So, it cannot recognize machine name that contains digits or uppercase letters : 
```
$ cat Vagrantfile
Vagrant.configure('2') do |config|
  config.vm.define 'app1'
  config.vm.define 'DB'
end
$ serverspec-init
...
Vagrant instance y/n: y
Auto-configure Vagrant from Vagrantfile? y/n: y
0) not
1) not
```
This pull request updates vagrant machine name regex to support name that contains digits or uppercase letters.