Symfony 4 support

Compatibility with Symfony4.0 & 7.2 (#84)

* Comptibility with sf4

* Limit tests to PHP7.2 and SF3.4

* Replace PHP7.2 by 7.0

* Tests passes with sf4 and PHP7.2

* Update README and add 3.3 to travis

* Fix composer with minimum symfony compatibility

* Remove symfony/symfony

* Add PHP5.5 & 5.6 in tests for SF3.3 & 3.4

* Remove PHP5 because PHPSpec4 only support PHP7

* Fix StyleCI