lazy loading serverspec

I'm using `vagrant-berkshelf`, but **Berkshelf::BerksfileReadError** occurred after `vagrant-serverspec` installed.
```sh
$ vagrant provision
Berkshelf::BerksfileReadError: An error occurred while reading the Berksfile:

  undefined method `path' for "https://api.berkshelf.com":String
```
This is a cause that `serverspec` was extend `Object`.

So, I moved `require 'serverspec'`.