allow Symfony2.4+ versions and do not use self.version inside require, test different Symfony2 versions

the test runs are a bit flaky:
https://travis-ci.org/BeSimple/BeSimpleSoap/builds/11510059
https://travis-ci.org/BeSimple/BeSimpleSoap/builds/11508488
https://travis-ci.org/BeSimple/BeSimpleSoap/builds/11508409