Simplified the Travis build matrix

Instead of testing all PHP versions against all Symfony versions, the
composer installation runs an unmodified composer.json for most jobs, thus
using the latest stable symfony components (which also ensure we don't
miss a dependency). Only a few jobs are running other Symfony versions, on
PHP 5.5.
This makes the build matrix going down from 14 jobs to 9 (while PHP 5.6
has also been added). Given that Travis limits the number of concurrent
jobs, this will help making builds faster.