Travis CI: Drop 5.3 as a build target

On second thought, I'd really much rather only target hhvm-nightly for now. On
the one hand, this is monopolizing / hijacking .travis.yml to a specific
Wikimedia project (HHVM deployment), but on the other hand, it's a big enough
project, so I'd like to ask you (dear reviewer) to consider it, as a short-term
arrangement. There are things like IRC notifications, overall build status,
etc. that are scoped to all configured builds and having 5.3 in there adds
noise. It also sows confusion about where we run our tests. People should
really turn to Jenkins for 5.3 testing. I think we're in the final couple of
months of hardcore sprinting on HHVM. I wouldn't mind opening this up after.

Change-Id: I0131667135ef3ca75d01ac5645dbc89bda306d98
Merge "Travis CI: Drop 5.3 as a build target"