Fix HHVM, prepare PHP 5.3 test for travis switch.

[aidantwoods]

Opt in to trusty early to fix hhvm, hold php5.3 on precise. Should open a bug to remove dist: trusty when travis makes the switch. 5.3 needs to be locked on precise though.

[aidantwoods]

Seeing as the travis builds take 20mins to complete, and travis allows 4 concurrent tests. Might make sense to split into build stages in groups like {5.3, 5.6, 7.0, nightly}, {5.4, 5.5, 7.1, hhvm, master}?

So test on oldest and last versions of the 5.x and 7.x branches first to get a pretty good idea of anything that'll likely fail if it were to fail between versions. If that's all good, test the interim versions and allowed failures (but if the first stage failed, don't run the next stage).

Just an idea for fast failing to bring down build times.

(Edited for coherency).