Merging a PR to a protected branch with approvers does not update the netlify-cms workflow

**- Do you want to request a *feature* or report a *bug*?**
bug

**- What is the current behavior?**
If the publish branch is a protected branch on GitHub with required reviews enabled, and you subsequently approve and merge the pull request in the GitHub GUI or CLI, the content remains in 'Ready to publish' section of the workflow.

**- If the current behavior is a bug, please provide the steps to reproduce.**
Set up a netlify-cms static site with GitHub backend, protect the master branch and create some content in netlify-cms to trigger a PR. Approve and merge the PR within github. Return to netlify-cms under the 'workflow' tab, the content is still in the column it was in on creating the PR, and doesn't look like it has been published, even though it has been merged to master.

**- What is the expected behavior?**
Whether the PR is merged in netlify-cms or GitHub GUI/CLI the content should not remain in the workflow

**- Please mention your versions where applicable.**

Netlify CMS version: 2.0.9