[BUG] Not Found error when running danger with GitLab CE without MR approvals API

**Describe the bug**
Our builds have started failing with the following error since using v10.6.2:
```
Error: { HTTPError: Not Found
at fn (/root/.npm/_npx/15/lib/node_modules/danger/node_modules/ky/umd.js:266:12)
at process._tickCallback (internal/process/next_tick.js:68:7)
name: 'HTTPError',
response:
Response {
size: 0,
timeout: 0,
[Symbol(Body internals)]: { body: [PassThrough], disturbed: true, error: null },
[Symbol(Response internals)]:
{ url:
'https://$GITLAB/api/v4/projects/$PROJECT/merge_requests/579/approvals',
status: 404,
statusText: 'Not Found',
headers: [Headers],
counter: 0 } },
description: '404 Not Found' }
```

**To Reproduce**
Steps to reproduce the behavior:
1. Execute danger with selfhosted GitLab CE without MR approvals API (for example 12.8.6)
2. Above error will occur

**Expected behavior**

Danger shouldn't fail, either by allowing the user to configure skipping using this approvals API or ignoring the error and carrying on without these API results. Or maybe there is a more elegant solution that I am not thinking of at the moment.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Your Environment**

| software | version
| ---------------- | -------
| danger.js | 10.6.2
| node | 10.23.2
| Operating System | swift:5.2 docker image (Ubuntu 18.04)
| Gitlab | GitLab Community Edition 12.8.6

**Additional context**
Also according to https://docs.gitlab.com/ee/api/merge_request_approvals.html#merge-request-level-mr-approvals this API will be moved to GitLab Premium only in 13.9, so expecting the API to exist/work might become problematic then as well.
Probably introduced in #1098.

Trace the GitLab merge request approvals request described in the report, especially the change introduced in #1098, and reproduce it against GitLab CE 12.8.6. Determine how a missing approvals endpoint should be handled, then verify that Danger continues without failing while preserving behavior when the API is available.

Written by the indexing model from the issue text.