Rollup merge of #43287 - kennytm:travis-ci-adjustments, r=alexcrichton

Rollup merge of #43287 - kennytm:travis-ci-adjustments, r=alexcrichton

Change Travis CI job order.

Reorder the job matrix to take advantage of the order how Travis CI starts them in rust-lang/rust. Plus other refactoring of `.travis.yml`.

1. Move the `$ALLOW_PR` image to the top, so pull requests will start testing as immediately after the build is started. Previously the `$ALLOW_PR` image starts 6 minutes after the build was scheduled.

2. Move the slow macOS images near the top, so they share more time with the rest of the faster Linux builds, which should shorten total test time (actually not much, about 7 minutes at most if this change does work).

3. Merged the `install` section of both Linux and macOS to make the `env:` section a bit shorter, and enable change 4 below.

4. Do not download or install anything if `$SKIP_BUILD == true`, which further reduces chance of spurious failure in the PR-CI stage (avoid the red cross appearing even if CI passed).

(IMO `$SKIP_BUILD` should not even exist: those irrelevant jobs should not start at all, but that would require travis-ci/travis-ci#2778 which has been rejected)