Vendor the latest Terraform so we can get the fix for ImportStateVerify
diffing on Removed fields.
```
export GO111MODULE=on
go get github.com/hashicorp/terraform@master
go mod tidy
go mod vendor
```
`govendor fetch github.com/hashicorp/terraform/...@v0.11.8`
I figured this was a good time to do it since we just had a release. I verified that everything still compiles and that a few tests are passing- I don't expect anything else to change, but if there are any weird failures that this exposes we'll catch them in CI with plenty of time before the next release.
* vendor: Ignore github.com/hashicorp/terraform/backend
This is to avoid dependency sprawl - e.g. vendoring AWS or Azure SDK
when we don't really need remote backend functionality
in provider code - it's core's responsibility.
* vendor: github.com/hashicorp/terraform/...@v0.10.0
