Reduce CI matrix for PRs

[dwijnand]

Fixes #4384

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• If the change is docs-only, [ci skip] is used to avoid triggering the build bots.

The ticket (#4384) mentions:

we should have some mechanism for triggering the extra tests in PR if a user wants them (e.g., a high risk commit.)

I'm thinking that for high risk commits perhaps calling the API to trigger an "api"-type build (as opposed to a "pull_request"-type build) would run the additional jobs. WDYT?

[phadej]

No way. It takes time, but CI catches problems especially with older GHCs.

I.e. this is not a proper solution.

[phadej]

EDIT: to expand. The master have been red for extensive amount of time, and no one had time to fix it. I won't make it any easier to accidentally turn red again.

Test locally, don't spam Travis.

[dwijnand]

Make sense. Then I take it #4384 should closed. I'll see if I'm able to help by fixing master.