HHVM & PHP7 support XML features, bugfixes and infrastructure improvements

This release includes many improvements and bugfixes, some of them are:
#580 HHVM compatibility
#301 Generate namespaced element on XmlList entries
#595 Fix xml-attribute-map for the xml driver
#611 Allow to not skip empty not inline array root node
#610 Allow to use custom serializer with primitive type
#397 Exclude annotation not preventing attempt to find public methods when using AccessType
#608 Composer is not able to resolve a dependency
#550 Generic way to solve setValue on a property which respects its setter
#584 Test on Travis always high and low deps
#583 Update Symfony validator and allow PHPUnit 7
#399 Added travis-ci label
#553 [doc] fix AccessorOrder documentation
#578 Fixed test suite on master
#577 Fix for a broken test: a missing (incorrectly positioned) argument