Use CI_API_V4_URL in GitLab if it is available

[beezly]

I'm sure there is lots of room for improvement in this change! Please suggest whatever you think is appropriate. I'm particularly feeling some hnng around the way I'm testing this change.

Basically, since GitLab 11.7 an environment variable called CI_API_V4_URL has been available (per https://docs.gitlab.com/ce/ci/variables/predefined_variables.html) which contains the information that we currently supply as DANGER_GITLAB_HOST.

I've left DANGER_GITLAB_HOST dominant because some people may have unusual configs where they have done things with hostnames that we don't expect and changing the behaviour may break things for them, but for new users it should now be possible to configure the CI side of Danger by just supplying a token.

[orta]

Yes! Perfect, this is great 👍

[peril-staging]

Thanks for the PR @beezly.

This PR has been shipped in v9.2.10 - CHANGELOG.