Get the Mergeable status of the PR before updating the
Status. This allows for the Status to be a required
Status for merging the PR. This allows us to use the
atlantis status as a Required status, which is the
configuration often used, since most deployments
want to ensure tf plans have all been successfully
applied before merging
Issue: https://github.com/runatlantis/atlantis/issues/453
Introduce new `mergeable` requirement, in similar vein to the `approved`
requirement. Ran `make go-generate` to update mocks accordingly.
This addresses issue #43.
