[travis] Auto-conf deps=high matrix line

[nicolas-grekas]

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

This generalize what we did for testing while working on major version:

• on master, the deps=high line checks out the last "number" branch for testing
• legacy tests are skipped when the last X.Y series is tested with master (X+1).0

[nicolas-grekas]

Travis config is ready. Once merged on 3.0, I'll cherry-pick the PR on 2.3 (doing it this way reduces conflicts)

[nicolas-grekas]

@stof , willing to review this one?

[jakzal]

@nicolas-grekas .travis.yml is really complex. It's not a subject of this PR, but what do you think of extracting most of its logic to a bash script? Could be more readable.

[nicolas-grekas]

PR rebased
@jakzal I'm convinced it would really help... Here, everything is in just one place, just read the file to understand how travis runs tests (even if it's complex I agree)...

[nicolas-grekas]

Status: needs work
Testing 2.8 should skip legacy tests, this is not the case currently

[nicolas-grekas]

Green on 3.0, now testing on 2.3: #16974