Add allow_failures to Travis

[sh0ji]

Addresses #992.

This should allow jobs to fail when they match certain criteria, allowing us to make some jobs non-blocking.

[nschonni]

wonder if it is possible to make it required for master PRs. That wouldn't fix the linked issue, but would mean that topic branches could keep going, but need to address the issues before merging into the mainline.

[sh0ji]

From what I can tell, something like the following would allow jobs that aren't part of a pull request (i.e. branch jobs) to fail, but the PR jobs wouldn't be allowed to fail since they don't match envs perfectly.

allow_failures:
  - env:
    - ALLOW_FAILURE=true
    - TRAVIS_PULL_REQUEST=false

I'd rather not implement that, though. The "allowed to fail" jobs still fail and maintainers can still ask contributors to resolve those issues if it is necessary.

And probably more importantly, we don't want regression coverage to be blocking, though. Getting the examples out is more important than ensuring 100% coverage.

[sh0ji]

I split the linting jobs into their own stage, which I may revert once I see how long everything takes.

[sh0ji]

Yep, splitting linting/testing jobs doesn't seem worth it. My original thinking was that it might be better to run the cheaper tests first (each linting job takes 40s-2 minutes), but the overall impact doesn't seem worth it, especially since there isn't a build stage.

@mcking65 this should be good to go once everything passes.

[mcking65]

Thank you @sh0ji and @zcorpan; Looks great! Excellent improvement!