feat(travis): pivot to container-based infrastructure

[bighappyface]

https://docs.travis-ci.com/user/workers/container-based-infrastructure/#Routing-your-build-to-container-based-infrastructure

[bighappyface]

This seems marginally faster than the legacy infrastructure:

Without sudo:false: https://travis-ci.org/justinrainbow/json-schema/builds/104474976

Elapsed time 1 min 58 sec
Total time 12 min 2 sec

With sudo:false: https://travis-ci.org/justinrainbow/json-schema/builds/104475549

Elapsed time 3 min 34 sec
Total time 11 min 29 sec

[bighappyface]

@localheinz thoughts?

[localheinz]

@bighappyface

Faster is better! 👍

[localheinz]

@bighappyface

But sudo is still needed, isn't it?

See https://github.com/justinrainbow/json-schema/blob/master/.travis.yml#L20.

[bighappyface]

@localheinz good question. I removed the CHECK_LINKS environment variable from the PHP 5.6 build to clear up some dependency issues that arose and started breaking the build. You had recommended something along those lines awhile back.

Now that the doc build isn't part of the build process we are good to disable sudo and get things up to date.

[localheinz]

@bighappyface

We could speed up the build even further by

• enabling caching of dependencies installed with composer (see https://github.com/refinery29/league-lazy-event/blob/master/.travis.yml#L20-L23, works best when authenticating with GitHub)
• generating coverage on one build only (see https://github.com/refinery29/league-lazy-event/blob/master/.travis.yml#L15-L16)
• disabling xdebug when we don't need it (see https://github.com/refinery29/league-lazy-event/blob/master/.travis.yml#L27)

In regard to coverage, maybe it makes sense to use https://codeclimate.com? It's free for open source, and it comes with fancy badges for displaying the code quality as well as coverage!

[bighappyface]

@localheinz +1

Feel free :)