[travis] reorder matrix + prevent "Killed" jobs
minor #13123 [travis] reorder matrix + prevent "Killed" jobs (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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[travis] reorder matrix + prevent "Killed" jobs

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The new --prefer-lowest matrix line fails randomly with ["Killed" message](http://docs.travis-ci.com/user/common-build-problems/#My-build-script-is-killed-without-any-error).
Lets try reducing concurrency on this one.
The matrix is also reordered so that the slowest lines are started first.

Commits
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b1d47e9 [travis] reorder matrix + prevent "Killed" jobs