travis should use the v3 import path not v2

[laverya]

otherwise tests of functions that do not exist in v2 fail

EDIT:
Also, without this pr tests are run against the current v3 code, not the code in the pr
This largely invalidates the point of CI

[laverya]

@niemeyer if the project is closed to PRs, just let me know
But leaving every pr here open without response is really depressing, especially things that should be approvable at a glance ☹️

[niemeyer]

Every PR is open because you are only looking at open PRs.

I appreciate your help, and your patience.

[laverya]

It's been more than three months since your last comment on an open pr / merge of a pr / etc
That's what I was thinking of 😢

Thank you for the work you do on this project!

[laverya]

@niemeyer I think I should point out the importance of this PR

Without it, TravisCI uses the submitted code as gopkg.in/yaml.v2, which means that unit tests will fetch the v3 package from github to run against. This means you can't use the unit tests as evidence for a PR not breaking things...

(and for this reason I've merged this branch into most of my PRs - they all have tests that rely on updated behavior, and so would fail otherwise)