[QA] zend → php53 in CI config

[Legoktm]

Hi,

In order to prepare for raising the MediaWiki core version requirement,
which'll happen at some point in the near future (I hope!), I have
renamed[1] all of the uses of "zend" in the jenkins and zuul config to
the more specific "php53". This will let us run say, php55 (or php7!)
tests for newer core branches, which are also technically "zend".

I haven't yet deleted any of the *zend* jobs from jenkins yet (or their
workspaces), so please speak up if you spot a problem, or think this is
a terrible idea, etc. I'll probably do that on Tuesday.

[1] https://phabricator.wikimedia.org/T103252

-- Legoktm