fix: force refresh when service version is reverted outside of Terraform

[Integralist]

Fixes #629

NOTE: I tested this locally by building the provider and following the following steps...

1. Create a new fastly_service_vcl service with two domains.
2. Delete one of the domains and plan/apply the change (creating service version 2).
3. Rollback the service to version 1 via the Fastly UI.
4. Run terraform plan and it should display the same plan as step 2.
5. Once plan is applied, then terraform show reveals service is now at version 3 and has the correct resources.

[kailan]

Looks good 👍

[jonnangle]

Thanks for this @Integralist!