Add support for Travis-ci container-based infrastructure.

[netweb]

Patch adds support for Travis-ci container based infrastructure to our .travis.yml file.
See more info at ​http://docs.travis-ci.com/user/workers/container-based-infrastructure/

Travis CI:

The build containers in our legacy build infrastructure have had 1.5 cores (with burst capacity) and 3GB of memory, the new containers have 2 dedicated cores available and 4 GB of memory.

Before: ​https://travis-ci.org/buddypress/BuddyPress/builds/44732927
After: ​https://travis-ci.org/ntwb/BuddyPress/builds/44776731

It's an obscure parameter name in the patch, but it is what it is sudo: false

Via ​#bbPress2736 and props to tw2113 please and thank you :)

[DJPaul]

Boone and I also had a very quick mention of it a couple days ago: ​https://wordpress.slack.com/archives/buddypress/p1418842435000673

[netweb]

What I added in Slack a short time ago:

​https://wordpress.slack.com/archives/buddypress/p1419290804001280
djpaul: I did read that chat here, as was just pointed out to me for the same ticket in WP Core, I forgot to mention "faster builds", and it is, though appears a little hit and miss per specific jobs

@djpaul: It has just been committed to WP Core if that sways the decision any :simple_smile:
​https://core.trac.wordpress.org/changeset/30987

[DJPaul]

I was just documenting the chat in the ticket in case anyone in the future looks into this change; though linking to irclogs probably would have been more future-proof.

Let's see if this speeds up anything.

[djpaul]

In 9262:

Travis: use Travis-CI's container-based infrastructure.

This may help to improve performance (how quickly it takes for the entire suite of tests to complete).
See ​http://docs.travis-ci.com/user/workers/container-based-infrastructure/.

Fixes #6078, props tw2113 and netweb.